You have a system with some problems, but those problems are well known and has 6+ years of playtesting and rules updates to support it. It is widely recognized as the most widespread ruleset in its genre and the most balanced by far, in addition to being *so* popular that it's part of the reason your game is being made at all. It is also the edition of the rules you claimed you're using to make the game.
There is quite a few people in the D&D community that would have a serious problem with trying to say that 5e is the most widely used, widespread ruleset in its genre. Most players I talk to prefer 2E and 3E far more than 5E.
The game is based on 5E, they will not be able to adapt every single aspect, no matter how much some scream and yell about it.
Personally, I prefer height advantage of cover advantage and here is a perfect example:
Have you watched "The Rock"? In one scene the Marines are on the upper floor of a shower room, the Seals are in the lower floor, but have significant cover. It was still a kill zone because that cover, could not counter the high ground. The Marines could literally just put their rifles over the wall and wipe them out, or even lob some grenades. Or hell even lob some tear gas down and cut their throats. It is the same principle as dry gulching in canyons. You may have some boulders or a scrub tree here and there, but that would provide minimal cover over a group of people 100 ft above you with their own cover.
Not to mention, yes there is circumstances where the enemy has the high ground in BG3, and it has a negative impact on your ability to fight. But they also provide a number of ways to counter that, from teleporting, the warriors pull mechanic, acid, fire destroying the platform they are using for high ground. These are all done without Barrelmancy or anything else.
I do agree, backstab should be limited to rogues. But that should be in battle as well. The rogue in a battle could pull off a dexterous move, jumping over the enemy to backstab. But also, when you try and run around them, you have the chance of that NPC turning and hitting you if they win the roll for it.
You may want to watch that movie again, the reason they are wiped out is because they have zero cover down in that death-trap. And the terrorists can easily fire from behind cover at the marines.
Regarding High Ground vs. cover: Now imagine I stand on top of a wall (3m high), and you are 10m away on an open field. Why should I have advantage to hit you with a ranged attack? Why should you have disadvantage to hit me? Especially with ranged weapons, you see me as clearly as I see you, you will have zero difficulty in your ability to target me and I will not have it more easy to target you. When I am standing on top of a castle wall behind a Firing port, then the main reason you will not be able to hit me is the cover, not the height.
Height does bring advantages, mainly increased range and it is often far more easy to get behind cover. If the height difference is very pronounced, then I just need to duck a little bit, and you won't be able to see me anymore.
So the Advantage/Disadvantage system in BG3 currently is just way over the top. But a lot of people have already pointed that out.
My personal preference would be increased range with bows and crossbows, not with spells, and that's it. If Larian wants to give more advantage to aleviated combatants, than I can live with a +1 or +2 bonus to hit, but not also a bonus to AC (aka a malus of -1 or -2 to the combatant on the lower ground).