The only problem I see with granting a flat bonus is that this might scale crazily in the higher levels. Advantage is really easy to get at higher levels.
You are stacking this with advantage and weapon enchantments - so you're now looking at roughly +8-10 to hit above bound accuracy.
Still, I'd take that over the current system.
And this is something people unfamiliar with 5e fail to understand. They are only seeing the portion where the characters have not come to their own yet. At higher levels, the combat will be ridiculous, almost comedic if the current system remains.
The best decision would be to remove height advantage and free backstabbing completely. Otherwise you are just pushing the problem to later parts of the game.