For what it's worth, backstab is actually way, way more powerful damage-wise then sneak attack is.
I think rather then just letting them do it as a sneak attack, which would make damage ratchet up insanely fast, you help them be more sneak-like. Make a few items that let them slip through shadows and such. In the 2e game I'm in now, our thief has a shortsword that, three times a day, lets her teleport in a cloud of dust to right behind someone's back - and since she's teleporting AND stabbing in one go, it's counted as a backstab. It lets her backstab in the midst of combat (and, for some enemies, brings them down with one hit), but it's not something she's actively doing every single turn.
I think rather then just letting them do it as a sneak attack, which would make damage ratchet up insanely fast, you help them be more sneak-like. Make a few items that let them slip through shadows and such. In the 2e game I'm in now, our thief has a shortsword that, three times a day, lets her teleport in a cloud of dust to right behind someone's back - and since she's teleporting AND stabbing in one go, it's counted as a backstab. It lets her backstab in the midst of combat (and, for some enemies, brings them down with one hit), but it's not something she's actively doing every single turn.