I think I have said this quite a bit, but I think Highground Advantage would be better suited as +2/-2 instead of True Advantage, since it is taking the place of the now partially-defunct cover/half-cover system. That flat bonus still gives higher ground meaning, but also makes it balanced and gives other tactical decisions more meaning and weight when compared to just "move characters to highest position and attack."
half Cover for those who don't know (pasted from DnD Beyond): A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.