@DragonSnooz I'm hesitant to argue for Backstab with finesse weapons only; Dex doesn't need to be any more overpowered relative to strength than it already is. Dex fighters would also benefit from this, not just rogues and dex rangers. My idea was to have some version of this feat for Str too (Choose +1 to Dex or Str, plus +to-hit effects on a backstab).
I like your idea that to get advantage, there can't be enemies within 30 feet. An alternate suggestion is to repurpose Tasha's "Take Aim" rogue bonus action ability, where you spend a bonus action to take aim and can't do any movement during that turn. This would ensure that anyone who wants to take advantage of high ground has to start and end their turn on the cliff edge, visible to all enemeies.
As other posters have mentioned, increasing characters' Proficiency Bonuses would probably be better than changing enemy AC since proficiency governs to-hit attack rolls AND spell/ability Saving Throws.