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.