D&D 5e is not designed to account for which direction a creature is facing. Backstab doesn't make any sense without that. "Behind them" isn't a concept. If you want it to be, there need to be more elements to facing; at a bare minimum, you need to be able to adjust your facing without moving. Making this feel right would not be a small task.
Advantage should be rare and should require significant effort to achieve. Most ways of getting advantage in the game require that you give something up in exchange. It might be a spell slot. Or you might spend a turn or two hiding and waiting for your target to come into range. Or your barbarian gets a little reckless and gives everyone else Advantage against them in exchange for getting Advantage themself. Or your front-liner spends their turns knocking enemies prone so the rogue can run in and stab them while they're down. If you're playing a game where flanking grants Advantage, it's a little easy, but at least you have to be aware of positioning. In general, if you want Advantage, I think you should have to work for it; Advantage is too valuable to give away for free. Making it a feat feels like too small a cost for something so valuable - it just means that everyone with a ranged character is going to take the High Ground feat because it's such an obvious choice, and then we end up exactly where we are, but everyone has one or two less attribute points.