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?
Hustle, I’m not fan of surfaces as is. We can definitely work them out in favor of the DnD rules. There’s the raw that says that you’ll only take damage if you end your turn over it. That would work. The other one is to add an extremely easy saving throw DC. But most importantly, If you miss the spell the surface won’t be created by it. So you guarantee that it’ll only be applied if you fail your attack save. The only thing that cannot pass is to have them in it’s current state. It’s just silly