Made it to the end...

+1 for the OP.

My take away is that the high ground rule only makes sense when you consider the lack of a cover system.
- 5th ed has 1/2 cover, 3/4 cover, and full cover.
- BG3 does not have the cover system but the High Ground sort simulates it for ranged attacks.

My feelings:
- please add the cover system updated for 3d environments (will need to cast more than 4 rays)
- reduce the high ground bonus to +1 or +2 to hit. (and no low ground penalty because cover has you covered now)
- add in the optional flank rules from DMG (2+ characters on each side of the target) with a +2 hit only. (melee needs fun rules too)