It's really challenging to homebrew the powerscale of a feat. The challenge is that in the current game-state verticality is everywhere, and jump makes easy backstab opportunities.
Using ASI as a baseline, the player would get a +1 to modifiers with ASI (+5% accuracy putting points into dex). Going back to the homebrewed feats: a +2 to hit is already 10% and a +1 to dex could be a 15% increase to accuracy, depending on the character sheet.
Focusing on backstab if the feat implies that it only works with melee dex weapons, that should be fine. It'd be popular for rogue and dex ranger, but other classes wouldn't consider it. I think advantage would be fine in this situation, it'd put rogue in a swashbuckler powerscale when using backstab (as rogue is now).
Bonuses from higher ground still would need more strict conditions to be met. Hypothetically if we commit to making high ground advantage into a feat... I'd do it like this.
Base-game verticality adv/disadvantage removed.
The feat would also apply a +1 to dex.
The feat would grant +2 to hit if no enemies are within 30 feet.
The feat would grant proficiency for all ranged weapons.