For a few of your points:

Questing - Never really my thing, but I'd say just do them in order of release, save the Cadwell's silver and gold achievement quests which is every main quest in 2/3 and 3/3 factions. That one takes a while. My order was quest/zone release order since One Tamriel. Story makes perfect sense, even though I haven't done anything in Orsinium or Craglorn. For TES fans who want to do quests (including dungeons, raids, and PvP story quests) in chronological order, there's a very extensive quest guide on the ESO subreddit.

Many people who main PvP do the Psijic Order quest line to get the abilities. It does take a while, though. Jewelry Crafting has no quest line, just the single tutorial. Both of these require Summerset. Antiquities also has no dedicated quest line and requires Greymoor. It's good for getting unique furniture pieces and a relaxing lifeskills activity. The other expansions unlock the Warden and Necromancer. Buying the expansions is a better deal than buying the classes solo if you want to play the quests. If you don't want to buy every single DLC, prioritize owning expansions and zones on sales and play the rest of the DLC content via a month of ESO+.

Builds and Dungeons - You can run any class on any race on any build. Most people don't care and the passives aren't enough to matter for anything other than min/maxing. As for builds, one-bar builds are petty popular with casuals and beginners. If you're trying to ease your way back into combat, there's quite a few of them on youtube. You can keep your current armor. A full set of Hundings Rage is still an all-round great set and the agility pieces you have will work for all overland and non-vet difficulty content.

Trading: I use ArkadiusTradeTools as my MasterMerchant replacement (MM is very resource-intensive) and TTC's website for finding specific items. Generally, pairing ATT or MM with TTC gets you all the data you need (still check TTC's website for certain motif pages and furnishings. Some of them go for crazy money but don't appear on trade tools because of low sales volume). The top trading guilds tend to be in the newest expansion zone, but the "most full" and highest-selling guilds are in the expansion zones > the capital cities > Rawl'Kha.

If you want to follow the story arc, see this thread. Someone posted a nice flow chart that might help

https://forums.elderscrollsonline.com/en/discussion/413807/what-order-should-i-do-esos-story-arcs-a-guide#latest

Overland Content can be beaten by just about any build out there. So, go have fun with what you have. Don't stress the min max thing. That mostly only applies to Vet Trials, Vet Dungeon Hardmode type stuff.

You can hone your rotation on one of the many training dummies in your house. Or, you may join a guild that has these dummies. A few of the dummies you can get for fee with doing some content.

Is your goal to game the economy? Or, just make some gold shedding your excess gear? I am in two casual guilds that sometimes get a Kiosk, sometimes they don't. I sell everything I list. A lot of overland set pieces sell in the range of 99 to 199 gold, even for trash items, like Melee weapons on Magic Sets. Reason is the Sticker Book which lets you collect sets without hanging on to the actual set pieces. (although I do still keep the favorable traited items on the sets I use).

So, if you just want shed excess inventory for gold, don't sweat the economy. You will make more than enough gold to finance your game activities doing overland content.