Advantage with high ground does not bother me so much. Tactical advantages require thinking about positioning, and I think it is fine, personally.
Tactical advantages DON'T actually require advantage mechanics, y'know? I can only assume you've never played D&D 5e, nor care to think how this affects the game beyond your most shallow immediate concern. Advantage is a strong buff and beyond the imbalance it causes on its own being so strong, it also causes a shit ton of imbalances to classes, feats, and spells that will require an avalanche of more homebrew to re-balance. Or just ignore it as seems to be Larian's modus operandi, and accept that many things will be broken.
Height advantage: Bonus to hit if above, penalty if below. I-Win tactic, but not guaranteed. Range itself is an advantage for ranged specialists, but a slight bonus like a +1 or +2 (perhaps even granting half-cover +2 AC/Dex save) would incentivize plenty without primitivizing the tactical combat you fool yourself thinking you promote. You don't actually promote tactical combat with simple "I-Win" tactics that will be the *only* correct first move in 99% of all scenarios. Add to this the peculiar/gamey implementation: Assuming you're slightly above your target; where you aim at the target might give you an overwhelming bonus or not. Aim at the head, you might not get it, aim at the feet and the height difference might be large enough to trigger the mechanic.
Flanking/backstab advantage: Bonus to hit if behind. I-Win tactic. You are free to move at the back of any opponent w/o them reacting. This is a constant, time-consuming reminder that turn-based combat (that I like) is a really crappy approximation of real combat and thus damaging to immersion. It is virtually guaranteed, so having to jump to the back every round just feel like a formality ritual that waste the player's time.
Larian have fallen into a trap of their own making, despite actually having raised early concerns over just this. They expressively stated they wanted to avoid a situation where there would be only one best tactic, yet have done their utmost to ensure this being the case with their lazy advatage homebrew. I'm frankly astounded over the level of obtuseness.
I agree with the other criticisms you raise however.
