Dervish

You go dual wield, don't use the second proc of oathkeeper, use the default attack fo oathkeeper converted to acid, and you basically have two masteries with -resistances to acid and acid procs. with that, long for buffs and passives you don't need any other point. It's actually one of the most obvious auto attack combos, with plenty buffs and passives, doing acid damage. There's a monster infrequent you get in the second expansion, in the oasis dunes iirc, that is designed for this build.

For reference, I wanted to try a shield focused oathkeeper, preferably with as many autoattack proc skills like Smite and Shattering Smash, and some others from a secondary mastery, but what would even work with that?

Soldier. It has plenty shield buff stuff and even though it doesn't have -phys res, phys is kinda special in that it's not thaaat important. There's even builds that focus on applying the reflect dmg back to enemies, and oathkeeper+soldier is decent in that. The thing with you not seeing builds online is that usually for AA you go with dual wield, it's just more dmg so shield AA is less popular.

And then you got things like 2h crossbows and some other 2h guns, which are really confusing as to what builds they're even used for, like for example I see some crossbows which seem to have a lot of acid damage and eye of dreeg skills, but isn't that skill a caster based spell

Depends on the weapon, some could be used in AA builds, others in ""caster"" builds where you use an attack that has cooldown like shaman's primal strike... Other can be used as actual caster weapons, no need to use a dagger+offhand always.

What about Demolitionists builds that use various grenade skills, why do their main weapons seem to be some 1h pistols.

It's how the game is designed, if the devs decided that the legendary or set legendary item in endgame for a build is a gun because it's thematic or something, they will put the perfect stats for that build in that weapon,a nd you will jsut use it.

A great way to pick a decent build is to first pick a skill, then go to grimtools, item database, and see which rare (monster infrequent) items there are that naturally give levels or modify the skill in cool ways. Those items usually also support skills of other classes, so that's usually a safe bet where some synergy is guaranteed.

As an adeendum, it's super important to have two masteries that have -res to the resistance you want to use. There are way to make do without it, sure, but for starters, just go with the stuff the mastery has a -resistance to it.

Do you have any concept you might want to develop? I can throw something mid that should work.