High ground advantage/low ground disadvantage isn't what I'd consider cheese, it's just a highly questionable design decision. Taking advantage of it by itself isn't cheese. But what pushes it into cheese range is the existence of bonus action shove, which further emphasizes control of the high ground by rewarding players with an opportunity for bonus damage via yeeting things that try to pursue your party. Not to mention that the existence of an ability that anyone can use which can outright result in instant kills when used at certain angles really shouldn't be a bonus action to begin with. There's a very good reason why the high ground/low ground advantage/disadvantage system as well as bonus action shoves don't exist in tabletop DnD, and BG3 is basically proof of why.
Agree with everything you said.
I would argue Shove is fundamentally unbalanced and cheesy as it is a normal action that somehow is unique for the player and that disregards physical realities. A halfling can bully an ogre freely with it, the kinetic energy is more akin to a cannonball than something realistic. But Larian's cheesy homebrew has unwittingly opened a can of worm that WILL require evermore homebrew or selective implementations. Feats like Sentinel+Polearm Mastery in conjunction with Shove will be able to damage and in effect stun-lock enemies.
Incentivizing high ground/flanking tactical movement is originally smart design. It devolves into Larian cheesy dumbfuckery by a complete disregard for balance. It is so over-incentivized it makes combat revolve around exploiting overpowered homebrew advantage - that furthermore makes a host of spells/class abilities (ie. Barbarian's Reckless Attack) redundant. Which again will require more homebrew or screw these over. ​
Flanking is piling on the cheese in further two ways. How the enemy AI consistently fails to abuse it (while they exploit height advantage so much, they almost break immersion by abandoning near melee encounters to scramble to faraway vantage points), so again it's mostly a natural movement that is "player only". Secondly, the flanking mechanic is virtually guaranteed for no/little effort (unlike height advantage). The way you move to the back of an enemy without them reacting is so extremely unrealistic it again hurts immersion. It simply strips the turn-based system naked as a bad approximation of real combat. Larian could easily have restricted this by ie. having everyone pivot towards one threatening enemy within range. Realism and balance restored. Cheese gone. But actual tactics and smart plays is an impediment to Larian's silly fun cheese.
D&D is a fragile eco-system, and Larian cheese is an invasive species.
