There are several major benefits to having toons in the game.

-Daily writs are incredibly efficient for generating revenue in the game, as well as Writ vouchers, and other rewards tied to writs and crafting.
-Once they are level 50, they will contribute to your Champion Points. They'll each have Daily bonuses for random dungeons and Battlegrounds.
-They each have a weekly Trial quest bonus.
-They can be used to mule items. In the late game, you should have either 200 or up to 215 inventory slots per character if you're willing to spend money.
-You can skirt negative penalties / bounties by swapping characters.
-Can unlock one random Nirnhoned armor piece per character by completing most of the Craglorn Main Quest

Important Warnings:
-If you use an autobanker, make a habit to visit your regular bank to deposit items before visiting a guild bank. Some guild banks have settings where you can deposit but cannot withdraw.
-Try to get Alchemy to tier 3+ (Level 20) ASAP to avoid wasting Corn Flower through daily alchemy writs.
-Keep high stocks of style stones related to each of your characters. Optionally, train them in each base-game style to slow down the process of using these stones.

Premium Currency / Monetary Costs
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*Purchase DLC/Subscribe to ESO+ for access to Jewelry Crafting in Summerset, Thieves Guild, Dark Brotherhood, Psijic Order, etc.)
Purchase Character Slots (Leave one slot open in case you need a burner character for speedrunning / testing purposes)
Purchase Assistants (Banker > Deconstructor > Merchant > Smuggler (Complete Thieves Guild DLC questline on one chartacter; optional) > Armorer (optional))
Purchase 3 specific non-combat pets that grant +5 inventory space each; might be more pets like this in the future

Optional
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Subscribe to ESO+ for Crafting Bag and doubled bank space

Not recommended
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Purchasing ANY skill line (Skill points are precious on toons, and these are pricey)
Purchasing skyshards (Fast and easy to collect after you upgrade your riding skills)
Purchasing riding skills

Recommended Add Ons in order of importance (Use Minion to add add ons faster than using the website):
MiniMap by Fyrakin (Settings allow to display Movement Speed as a velocity or a percentage value. Used this for testing)
Dolgubon's Lazy Writ Craftor
DailyProvisioning by @Marify
*DailyAlchemy by @Marify (Seems to burn through valuable resources if you lack knowledge / run low on less valuable resources - use with caution)
BeamMeUp - Teleporter (Has an automatic waypoint discovery option to automate traveling to friends/guildmates)
Dustman by Ayantir, Garkin & iFedix (Junks junk items and expedites selling process)
Character Knowledge by @code65536 (lists which characters on-account know/don't know highlighted item)
Autobanker by r4cken (For automatically depositing important items/resources such as currencies or survey reports/maps, writs, etc. to the bank)
Votan's Keybinder (Make account-wide keybindings at once)
WritWorthy by ziggr
Ulrich's Skill Point Finder
Dynamic CP (I think it allows you to save and apply CP sets easily)
Mail Auto Delete by Pachvara (Loots/deletes generic mail rewards)
Jack of all trades by @CyberOnEso & @MMasing (Allows for dynamic switching of CP skills with a limit of one change every 30 seconds due to game code)
Sous Chef by Wobin, Baertram, CrazyDutchGuy, katkat42, Rhalyf) (Sorts ingredients)
AutoBind by ShinyBones (Automatically binds gear you have not yet collected as you loot it)
Lost Treasure by CrazyDutchGuy, Scootworks, et al. (Displays locations of Treasure / Survey reports on map)

Optional:
Azurah by Kirb, Phinix, Garkin (Reposition HUD elements)
FCO Lockpicker by Baertram (Makes Lockpicking almost trivial)
Inventory Insight by manacortex, AssemblerManiac
Keybinding: Log Out by @code65536

Now that that's out of the way, it's time to get on with the guide itself.

Gold bottlenecks:
-Once you have 3,234,600 Gold free to spend, consider buying all pack upgrades for each toon.
-Food recipes will cost 379,080
-Drink recipes will cost 379,080

Horse Riding Skill upgrades will cost 4500 gold per day, but that will barely cut into your profits as you level each character. After 180 days, you will have spent 810,000 Gold on Horse Riding Skill upgrades

ᴥToon Pre-Requisitesᴥ

When should I start making toons?

Whenever you feel comfortable with it. Keep in mind they're costly. Recipes alone get expensive since you're effectively multiplying each 18 times, in my case. Anything you need to do will have to be done 19 times total by the end of it. Focus on your main, then move on to toons. I started back when I was ~380CP and had ~270k gold and ~120 skill points. You can also subsidize costs by holding out for recipe drops or motif drops, but these also tend to be pretty accessible through guilds or guild traders, usually for cheap (atm I wouldn't recommend spending more than 500 gold per basic race motif).

Consider buying houses as well. These are account-wide. CP is also account-wide, even for characters under level 50.

Companions are effectively account-wide in terms of XP, skills, and equipment, though they have to be unlocked for use by doing their introduction quest per-character.

Character knowledge affects Master Writ chance, to a small degree, and with diminishing returns (most likely). Note that singular pages do not contribute to character knowledge - you need every page of a singular style on a single character for this to count. I would recommend only learning recipes and Enchanting stuff - maybe Alchemy as well on alts. Motifs just aren't that important for how much work they require.

ᴥPlanning Phaseᴥ
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Character Race and Class are non-vital. This guide should allow you to optimize your characters movement speed without relying on class bonuses or active abilities.

My personal build:

Since we're using these toons mostly for economy, we're less concerned about combat optimization. I strongly recommend focusing on crafting skills and trickling in Dual Wield actives and actives for a second skill-set as you go. I find Dual Wield to be the best weapon type for DPS in the game, and I'd recommend going healer for a backup, so Restoration Staff, though you could consider focusing more on a Class or tank/bruiser build instead.

Due to the nature of this duality and jack-of-all-trades mentality for toons I evenly apply stamina and magicka upgrades with each level, ignoring health entirely.

If you wish to use the Armory feature, a trick to consider is to have a blank build that you can go back to, to build any sort of way you wish without ever having to pay for a respec.

Though counter-intuitive to most, I will save random dungeons until level 50 for four reasons:
1. Maximize CP gain.
2. Minimize queue times.
3. Occasionally complete Undaunted quests for keys.
4. An abundance of high-priority issues in the early game.

I made a personal guild in hopes of eventually being able to make a private guild bank, and joined a couple of utility guilds - Beam Me Up and a trading guild.

I don't recommend putting skill points into trait research speed. Trait research on alts should be done casually.

I also don't recommend bothering with Training trait items. You will eventually get everyone to Level 50 naturally.

Make sure to take Meticulous Disassembly and max out inspiration gain in the CP upgrades. I then recommend maximizing gold, and eventually increasing Mount Speed, resource doubling chance, and resource gathering speed.

ᴥIntroduction Phaseᴥ
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-Change Keybindings / Quickslots / Settings
-Disable help pop-ups
-Apply CP sets
-Start first quest
-Complete first delve + sidequest for XP, collect Skyshard. Skip boss.
-Continue/complete first quest you picked up.
-Fast-travel to your first home in town, complete House quest.
-Complete starter quests in-town (Guild Finder, Mages Guild, Fighters Guild)
-Complete introduction quest in Cyrodiil (Do not skip!). Unlock Continuous Attack in the Assault skill line for Major Gallop (+30% Mount Speed)
-Grab The Steed Mundus Stone
-Pick up Battlegrounds quest just in case

If you skipped the Cyrodiil quest (which I did accidentally a few times), complete the first battlegrounds quest to reach Alliance Rank 3.

At this point, you'll want to reach level 5 with each character. A cheap way to do this is by using automatic waypoint discovery with the BeamMeUp map.

Once you are at level 5, get certified in the six base-game crafting skills, then go to Summerset and get certified in Jewelry Crafting.

ᴥBase Prioritizationᴥ
-Login Bonus
(-**Holiday Event on Main)
(-Horse Training on Main)
-Daily Writs on Main
(-Trait Research on Main)
(-*Obtain one Daily Siege of Cyrodiil Merit from The Imperial City Siegmaster Crates on Main (Can grind multiple chests per day))
-Seals of Endeavor on Main or Toon
(-*Weekly Trials on Main) (Until 100% completion)
(-Horse Training on Toons)
-Writs on Toons
(-Daily Random Dungeon on Main (Stop after collecting all Skill Points or CP = 3600))
(-Daily Random Dungeon on Toons (Stop after collecting all Skill Points or CP = 3600))
(-Daily Random Battlegrounds on Toons) (Unless Alliance Rank 50 or CP = 3600)
(-Daily Random Battlegrounds on Main) (Unless Alliance Rank 50 or CP = 3600)
(-*Weekly Trials on Toons) (until 100% completion)

*optional
**seasonal

General "Don't Forget" Tasks
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-Do scrying leads before they expire.
-Do most treasure maps when you are running out of inventory space.
-Sell master writs you can't do at a guild trader to free up inventory space.
-Do survey reports when you run low on materials.
-Refine raw materials on main with full builder skills.
-Get Alchemy to tier 3 (Level 20) on all characters ASAP to avoid using Corn Flower in daily writs.
-Maintain high stocks of base racial style stones.
-Get Ring of the Wild Hunt from Antiquities before toons get to level 50.

ᴥSkill Prioritizationᴥ
-(2 Skill Points) Intimidating Presence (Guild > Fighters Guild) and Persuasive Will (Guild > Mages Guild)
-(12 Skill Points) Active Dual Wield Skills
-(2 Skill Points) Alchemy - Solvent Proficiency Level 3
-(3 Skill Points) Alchemy - Chemistry
-(6 Skill Points) Provisioning - Chef + Brewer
-(15 Skill Points) Hirelings (Enchanter > Forager > Miner / Outfitter / Lumberjack)
-(15 Skill Points) Crafting Extraction Skills (Jewelry > Runestone > Metal / Unraveling / Wood
*(2 Skill Points) World > Soul Magic > Soul Summons
*-(4 Skill Points) World > Legerdemain > Locksmith
-(~12 Skill Points) Secondary skill set actives
-(10 Skill Points) Dual Wield passives
-(7 + 2 Skill Points) Medium Armor Passives (Athletics optional - see Movement section below)
-Secondary skill set passives
*(9 Skill Points) Racial Skills
*-Guild > Thieves Guild > Haggling
*(4 Skill Points) World > Soul Magic
*(4 Skill Points) Guild > Undaunted > Undaunted Command + Mettle
*(4 Skill Points) World > Legerdemain > Trafficker
*Class Passives that give constant bonuses
*(4 Skill Points) World > Legerdemain > Improved Hiding

Never Recommended:
-World > Scrying + Excavation
-Keen Eye Skills
-Research Skills
-Improvement Skills
-Guild > Thieves Guild; Swiftly Forgotten, Clemency, Timely Escape
-World > Legerdemain > Kickback
-Vampirism

*optional

ᴥEarly Game Goalsᴥ
-Get a Companion (Sharp-As-Night recommended) + Collect Public Dungeon Skyshard (~1-2 hours)
-Draw them like one of your French girls
*-Complete "The Queen's Decree" in Summerset, and unlock the Psijic Order skill line. (~1-3 hours) (See below)
*-Level Legerdemain (see below)
-Do Public Dungeon bosses + skillshards (minus the six most recent, hardest public dungeons from Blackwood to Apocrypha)
*-Complete Craglorn Quests (Random Nirnhoned Armor for research / sale)
-Do Main Quest
*-Do Thieves Guild
*-Do Dark Brotherhood
*-Do Zone Quests

*optional

Psijic Order
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This will be a bit of a mind-numbing grind, but it is technically best to do early as you will maximize your chances of coming across Psijic portals and the loot within them. We do not need the other skills in this line, however. MAKE SURE not to leave the delve before bringing up the Psijic Map of Summerset or you will be required to return to that location. The only skill points you will get will be from skyshards located between wayshrines and rift locations.

Level Legerdemain
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While going through public dungeons on main I found these to contain an abundance of chests, and at times these chests were more hassle than they were worth due to absurd enemy spawns. 4 points into Lockpick makes looting these chests very convenient.

The route I take for leveling toons in this stat is to do writs in Vivec City, do horse training, then go to southern Craglorn and do the notorious apple-stealing method. Make sure to loot everything, build up a healthy amount of stolen stock, then head to the Refuge nearby; sell treasure, and launder useful items like ingredients and lockpicks. Forage along the way - Nirncrux now drops everywhere in Craglorn.

If you get caught looting apples, never die to the guard, and never pay him either. He's easy to get away from. If the civilian attacks you, let your companion kill him then loot his body as well.

My strategy to skirt any bounties is to do a run of looting and fencing/laundering to the cap, then looting again, then moving to the next character. Do a full round of this, then go back through and take each character, now bounty-free, to Vivec to do writs. You should end up doing 2 logs and 3 looting runs (be careful on the final) from Craglorn to Vivec, and 1 log and 2 looting runs from Vivec to Craglorn. Don't bother upgrading transactions as this will barely sustain 100 interactions per day per character.

The route I do is a 2-parter that switches between Craglorn and Vivec City from day to day.

Don't Forget:
-Collect nearby Skyshards
-Collect nearby lorebooks
-Visit nearby wayshrines
-Collect Ore/Wood/Powder/Plant nodes (whenever in Craglorn)
-Never pay guards
-Send me your Sharp-As-Night fanart

Then, after Level 50 you should consider focusing on Random Dungeon queues with all three Undaunted Quests active every day, rotating characters and eventually getting all/most dungeons completed on all characters.

ᴥMovementᴥ
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Movement speed is crucial for optimizing your toons. Ideally you'll want to passively have 200% movement speed at all times, and will also want maximum mount speed (which is completely separate). Movement speed has a hard cap of 200%, and mount speed has a soft cap of 287% (No confirmable hard cap).

I can't think of a better way to do it than the following

+10 from Celerity** (CP - Fitness)
+15/45* Ring of the Wild Hunt (Requires level 50, Antiquities hunting, and many Transmute crystals)
*21 (7 * 3 Swift Trait on jewelry, default trait on RotWH)
+10 to +26 = +10 + 16 (***0.091 * 7 Divines Armor Pieces)

Not Recommended
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+20** from Steeds Blessing (CP - Craft)
Any active skills
+30/+15 Major/Minor Expedition (from sets or skills)

Mount Speed (115% default)
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+60 from daily training (1 per day)
x1.3 (+30) Major Gallop from Assault skill line
**x1.1 (+10) Gifted Rider CP - Craft
245% walk, 287% sprint


*Out of combat
**Slottable CP Skill

This combination should bring you to 201% which will get capped at 200% walking speed. This will drop in combat, which will allow you to hit the cap again anyway if you sprint.
Thank you to Oreyn_Bearclaw for providing extremely helpful information and analytics on movement/mount speed in the game.