I agree with all of this, especially the height mechanic. Your characters should be scrambling for cover (if squishy mages) and not scrambling for high ground out in the open.
I've been planning on doing a write up on this. It is a big issue that the enemy AI is set to go for high ground and mages are incentivized to go for high ground. The game asks the player to put their mages in bad positions. Spells and Bows having equivalent range is also part of the issue.
Mages innately have a bunch of tactical decisions in 5e to make, which spells to use and where to be to zone the enemies safely.
I can understand that fighter and ranger was lacking tactically and high ground / low ground might be interesting for them... but it's a problem treating magic like it's a crossbow bolt. Fighters, Barbarians, Rogues, and Rangers have the armor and weapons to deal with enemies at close range consistently. Mages usually have to use a spell slot on a close range spell.
Mages should not have an incentive to go where their enemies are going. And it doesn't make sense that magic would be granted advantage/disadvantage just for elevation.