On the one hand I get the criticism. For example why is 33K armor 50% mitigation? You only know this from having been around. There is no reason for ZOS to be so obtuse about that.
On the other hand this must be partially due to ESO being spaghetti code. I bet even ZOS don't truly know, e.g. because their software department has turnover and knowledge disappears. What we do know is that the calculation is more complex than every factor being multiplicative. If all damage and mitigation factors were multiplicative, you would feel a power increase proportional to the tooltips. However while that should work on the mitigation side, having all multiplicative damage factors would most likely exacerbate power creep. Not that I reckon ZOS have been thinking about that clearly and consistently over the years. Thus we've ended up with a mess of a calculation where, for example, the 10% block mitigation from Defensive Stance is additive with other block mitigation factors and can measure the same as 27% multiplicative would be, depending on what baseline you're comparing it to. The reason I mention this is to illustrate that even if things were expressed clearly in percentage terms, that can still be very deceptive. It's not merely the armor and crit figures, which you have to convert to percentages yourself, that make things obscure. It goes much deeper than that.
You don't say what content you play. If you are talking about being a PvE DD, and you want to make your own builds, the answer is really quite simple. You parse. The handed-down knowledge, the meta, is essentially created by endless parsing. By people looking at the Combat Metrics addon in detail. Sure, if you're Skinny Cheeks et al, you also make spreadsheets. Mostly what I see my guildies doing, though, is they obsess over the uploaded encounter logs after a trial. You can clearly see stuff like the big damage boost templars get in execute, or the times when heavy attack builds did crazy OP AOE damage in such logs. This is a public web site, as far as I know, e.g. esologs.com, I think. You can see what other people are running and where their damage is coming from in actual content, e.g. trials. As a healer you can also work out what skills are the most effective, and you'd be surprised how much tanks rely on self-healing.
After 2 1/2 years of farming, I completed my stickerbook and can now recreate and destroy any build "at cost". I also bought a trial dummy for my own house. I did this, because I enjoyed the journey and because I enjoy build-crafting. I'm not saying you have to do that. You could go to the PTS for parsing, then farm only what you need on live. For better or for worse, this is part of the game, isn't it? If you want to be competitive, then the testing and work that goes into that is part of the fun for some.
Running someone else's meta setup isn't necessarily the best solution for yourself. You're right in wanting to make your own builds, in my opinion. I parse around ~92K on my Arcanist for example. That is off the pace. Top players do significantly better. However it is above average by PUG standards and I'm happy with that. I compromise my builds for easy rotations, for survivability, and for not having to constantly quaff potions. What I have noticed, in doing so, is that meta approaches do not work 100% for me. For example Order's Wrath is ranked quite highly by some content creators. However I have found that's probably only true with a to-the-teeth damage rotation. If you skimp on damage skills and on your rotation, I think you get more value out of proc sets, such as Whorl of Depths or Pillar or Nirn. That said, on Arcanist I run a straight up stat set, e.g. Ansuul.
As to some concrete questions. How much of a crit damage modifier do you need? This is highly dependent on your group. In general, running the Thief mundus is best and you won't come near the crit damage cap on your own. The only time where that cap may come into play is IMO trials. Not PUG trials, though. You have to be coordinated to get there. People have to have good Warhorn uptimes and not overlap those. You may need an EC Necro in the group or a tank with that new tank set. In other words, this is not something you worry about. If / when you start to worry about that is when you are in a vet trial progression group, in which case you still don't worry about it, but the raid leader will typically work out the group compostion and they will prescribe a build for you, because your build needs to be coordinated and not overlap with the rest of the group.
If you're running in PUG (random "pick up group") (trial) groups, builds don't matter all that much. They only come into their own in coordinated groups and, most of all, when you have a good rotation, which many PUG players don't. People like to brag with their knowledge and pretend this isn't true. They may have issues with you not wearing perfect gear, but unless you're wearing outright garbage, that's all nonsense. The single most important thing you can bring to a PUG is mechanics knowledge and a good skill rotation. Everything else is secondary, because everything else genuinely only starts to matter in coordinated play. Wearing the right build in a PUG, in so far as people ask or can verify that, basically just raises the trust people have in you, like a badge that you're belonging to the right club. For example it is customary for one healer to wear SPC / Pillager and another RoJo. Does the RoJo healer get the heavy attacks in and make the most of that? Do the PUG (or training guild) DDs, struggling with their rotation, truly benefit? "Naaahh", on both counts.
As to a crit modifier in PvP, you cannot have enough. It's impossible to reach the effective cap, because people have crit resist. That said, the only archetype that springs to mind, who would truly care, is a Khajiit ganker nightblade.
As to what your crit damage modifier is, it's on the Advanced extension of your character sheet.
In regard to penetration. For solo content it can be worth investing into it, up to ~9K unbuffed / ~18K buffed with Major and Minor Breach for bosses, if you can find that in your build. Sources include light armor, being an Arcanist, certain armor sets, and a Sharpened weapon. You would test on a normal target skeleton, not the trial dummy, to assess the difference. For trial content, however, don't invest in penetration. Not really for group dungeons either. Up to 7K could be useful, but your tank should take of the rest at a bare minimum, e.g. the tank should provide at least 11K on bosses. In PvP, on the other hand, you can't really have enough penetration, albeit temporary sources are as effective as permanent ones, e.g. Balorgh or Corrosive Armor.
All of that information is in the character page in your menu. They added it like 3 years ago.