(Keep in mind I still have mixed feelings about low ground Disadvantage, I would like to see High Ground changes first).
I'm familiar with the rules for cover, but Baldur's Gate 3 already has collision detection. Which is something tabletop doesn't have. The player or enemy can move backwards and get out of range, or hide. The attacker is forced to use their movement to target their enemy.
When talking about low ground Disadvantage we do need to consider all the terrain benefits that a 3D game can provide. Cover in tabletop has always been a composite for that. So should characters in the game get double the benefit of terrain? That is the question when considering low ground Disadvantage. (Side note: I am implying that the current game state almost triples the value terrain can provide.)
Lastly, a -2 to -5 is the same as saying Disadvantage is fine in these situations. That's the expected value of Disadvantage depending on AC. Advantage/Disadvantage Table
According to you, collision detection makes up for the lack of the cover rule, so why not make up for the advantage/disadvantage rule? Apart from the other benefits of high ground that I've already mentioned.
But the point about -2/-5 is that, it would be more realistic, because with collision detection, you can't hit "just the head", so this cover rule would allow the attacker to do that, but with the difficulty of increased enemy AC.