The only problem I see with granting a flat bonus is that this might scale crazily in the higher levels. Advantage is really easy to get at higher levels.
You are stacking this with advantage and weapon enchantments - so you're now looking at roughly +8-10 to hit above bound accuracy.
Still, I'd take that over the current system.
I don't really see the problem here? 5e RAW has Cover rules which grant +2/+5 to AC, and these bonuses don't pose a problem at higher levels. So why would the smaller +1 or +2 high ground bonuses, which in BG3 would replace these cover mechanics, be a problem? Sure, cover grants AC bonuses whereas BG3 high ground grants to-hit bonuses, but tomato tomahto.
Mainly the sharpshooter feat. Sharpshooter negates cover and now you have an extra bonus on top of no cover. I know SS is not in the game yet but GWM is and I suspect we haven't seen SS yet because Larian is well aware of how powerful it will be with advantage (it's already a very good feat to begin with). They will have to tweak SS if they want to implement it in BG3.
Also cover and height bonus, although may do the same thing, they affect the fight differently. If a foe wants to retain cover, they generally have to stay in their position or risk losing cover. That tactic won't work with height advantage. There's no turtling of mobs.