Isn't the purpose of non-parallel walls, obviously not to get rid of reflections since it can't do that, but to help make sure that what comes back to you has A) gone a more circuitous route which reduces its energy more (particularly if it's hitting absorbers and diffusers along the way) and B) delays it more help ensure that what does come back to you is less of a direct reflection of what is coming at you from the speakers, which is where the biggest problems arise.

On the issue of thickness, that's sort of less of an problem if you are building, since you can recess it into the walls. So you don't have to have it sticking out into the room necessarily. Build the actual room, then you can fill the whole back and front walls if you want, and put a secondary 'wall' over that, which may be mostly fabric cover. You can put a diffuser on the rear as well if you want.

You can do similar things with the corners, where it doesn't look like treatment, it's just a non-square corner that looks like the rest of the wall (or looks tastefully not like it if you want.)

There are various ways to do it and make it look nice, while still getting, maybe 6" or 8" or more of thickness. That's not the end all of treatment obviously, but it would make a tremendous difference compared to the average person's home theater response.

Obviously, if you have the bucks and experience and tools, you can super-tune a room. But there's a lot of info out there, and if you learn the diagnositic tools and experiment, you can make a huge difference on your own. You can move the materials around and make comparisons before you commit, so you don't have to guess or even theorize (though the theorizing can guide your experiments clearly.)