Display/hide by mesh should be a feature, ten versions ago

Workflow that works, quite shortly, is just to rip everything apart in modelling program, paint it in substance, and then attach together again. with stuff like closets or trunks and such, you also have to move lids/doors/etc quite far away, otherwise ambient light will bake "darkness" into them.

Man, I’ve been digging through the internet and praying to the AI gods just to figure out how to do something basic that the software should already have - but nope! All because I want more freedom to work with multiple objects in the same texture set and NO ONE that is reponsible for the development seems to work with it. hahah
So yeah, I totally get your feeling!

At the end of the day, you either roll with what you’ve got or jump to another software. I’d rather stick with what I have, ‘cause every time I hit a roadblock the software won’t fix, it’s way quicker to come up with a "around-the-corner nonsense" than to beg the aDoBe TeAm for an update or learn an entirely new software;
especially with deadlines breathing down my neck.

I recently discovered the UV projection type for paint layers. The default projection setting is tangent, I believe. The UV protection setting was a game changer in allowing me to paint behind and around other objects, without those objects interfering with my brush. It doesn't solve all of your problems but might be helpful for some situations you describe. Note, that it's not useful when using brushes that depend on tangency, like the new pucker brush.

"Frustration" is a soft word for it. Try to texture innards of a refrigerator or a cupboard and problem would be clear in seconds. Plus, the way camera works in Painter is very painful, there's no key (that I'm aware of) to refocus camera view, and if you actually scroll inside your geometry, camera starts acting quite weird the closer you get to object pivot. The way free-fly camera works in Unreal Engine would be a really good method to adapt about this, I think.

Only low-tech way right now to access hidden geometry is to literally tear your model apart in 3D software before painting, right now. Which feels really needless when much more user-friendly solution is so obvious.