@etonbears I appreciate the detail you put into your response. I'll try to respond in kind.

I've included my responses in the spoiler boxes to clutter up the thread less.

1.) Height/Backstab Advantage vs numerical bonus
Yes, you are correct in what I'm saying. Currently in BG3, Height/Backstab are equivalent to ~+/-5 bonuses that don't stack and I'm suggesting that it should stack with other abilities (as a +X bonus, where X is between 1 and 5)
I agree with you: it makes sense that height gives benefits. You can see people more easily from high ground and will be harder to hit because of cover. I'm not opposed to these benefits. But I do think it is necessary to make these height bonuses stackable with normal sources of Advantage, or else a lot of the game and character's abilities become irrelevant. See the list of 56+abilities made by the OP.

1b) Backstabbing: In your most recent response, you use "unsighted," "backstab," and "flanking" interchangeably. These are all different mechanics in 5e
--You can Hide, preventing the enemy from seeing you = unsighted. This grants advantage to attack rolls. I agree with this being in the game. You have spent a significant resource (Bonus Action) to gain a bonus (Advantage)
--There are optional flanking rules, where you have allies on either side of the enemy. This is not implemented in the game. I would be fine with this granting a bonus (again, Advantage is too strong. I'd suggest a +2 or +3 bonus). You have spent a resource (significant positioning requiring coordination between 2 PCs) for Advantage.

Neither of these are what Larian has implemented as "backstabbing." The current problems with Larian's implementation are that a) It costs basically nothing to circle around an opponent. b)Advantage is too strong and doesn't stack. See below for my argument re:moving being ~free

2.) Movement as a resource
Yes movement is a technically resource, in that each turn you have limited movement. But, often there is no cost associated with moving, because you don't lose out on something like you do if you spent your action not attacking. (Also climbing a ladder costs literally 0 movement!)
---For example, if your fighter is next to an enemy. It is almost always the correct decision to circle around to the back of the enemy, getting this "backstab bonus." The fighter then still has their bonus action and their action. They haven't really lost anything.

In some cases, yes there is a cost. However, these situations are rare, negated by other Larian decisions, or have very small costs
---You would have to leave the threatened area of another enemy. This situation is somewhat rare. This would require multiple enemies to be near your fighter, none of which you can already backstab without moving. In addition, this cost is reduced by the ability to jump+disengage as a bonus action, ~always enabling you to do so, especially given that many Fighters don't have bonus action options anyway.
---Your example: "you must consider where other enemies are and can go". I'll admit, this can be a cost. But, as you said, 5e's has very lenient movement rules. This makes it much less likely that your decision to circle behind/gain height will actually put you in a worse position. The enemy could probably already move to gain whatever position they wanted. Again, "flanking" does not exist in BG3, so you don't have to worry about being surrounded.
--The biggest risk is being shoved off high ground by an enemy. This is significant. Which is why I'm more favorable of height bonuses than backstab bonuses. Risk&Reward

Also, I believe the opponent won't circle around you for backstab. They'll try to get in a position to backstab if they're not already next to you, but they won't circle around you. This might be wrong though...but if true, then backstabbing benefits don't apply equally to all actors

In summary, while there can (sometimes) be cost to moving, there often isn't or the cost is small compared to the Advantage gained.

3.) Feats/abilities/spells
The list of 56+ things is in the first post of this thread. Does it really matter what exactly they are? The point is that there are so many options that are made irrelevant by easy sources of advantage. Some of them are integral to classes/spells e.g.,
--Bard's spell Vicious Mockery, giving an enemy disadvantage to hit, is almost always used in PnP. If I'm on high ground, the enemy already has disadvantage against me. Why would I viciously mock?
--Barbarian's Reckless Attack grants you Advantage on all your attacks this round in exchange for enemies getting advantage on you. Why would you use this if you can just circle around an enemy for advantage?
--Faerie Fire grants advantage on all attack roles against a target, and is incredibly powerful for that. But why would you cast this if instead, you can just circle around an enemy/get height advantage?

I agree with you that, with infinite long rests, spells can be used more freely. But that is not the timescale I'm talking about. I care about a single combat, a single turn. In a single turn, would you rather:
a) walk up stairs/circle around an enemy to gain Advantage, where you still have your Action and Bonus Action?
b) spend an action (cast a spell) to POSSIBLY (they could resist) allow your allies/you to have Advantage on their attacks? In this case, you've used up your Action, and thus your turn is over
My point is that Option B is strictly worse than Option A, because you can achieve Option A will less resources used.

tl;dr: My point is that Advantage (the mechanic) from height/backstab is too powerful and too easily achievable. It makes lots of other parts of D&D irrelevant, and the costs for getting height/backstab are often small given 5e's lenient movement rules.
That said, I do like Larian's environments involving height. In this, we agree ^_^ I just think that a +2/-2 bonus for height (or even a +4/-4) that stacked with Advantage/Disadvantage would be much better.
Similar for backstab: change it to a numerical bonus that stacks, and maybe change "backstabbing" to "flanking", where you have to have allies on opposite sides of an enemy. That would involve a higher cost and more tactics, as you'd have to maneuver 2 people to positions in the thick of melee. Risking getting backstabbed themselves or AoE'd