Really well done post OP.

Surprised long rest spam and spell slot economy wasn't mentioned as that's definitely a big factor. You don't have to be conservative with spells since you can get all your spell slots back after every encounter, which leads to casters outperforming martials easily. I think if they can put some form of a limit on resting, going back to 5e AC and hp values would be ideal.

I think elevation works for a defensive advantage like cover(as in +2 AC and dex), but not as an offensive advantage. Backstab should require flanking or stealth.

Now for surface effects, I'd really like for those to remain in the game as they add more complexity to fights. Would adding saving throws for surfaces and maybe making concentration harder to break be enough to balance out the surfaces?