Sneak attack, hello? There are no spells or background abilities that let a non-rogue sneak attack. If a BG thief happens to start a combat round un-hidden, bonus action hide, sneak attack, bonus action hide again. Already hidden? Not in good position? Bonus action dash, sneak attack, bonus action hide. Two bonus actions is huge. If BG thieves lost the ability to pick locks and disarm traps, I'd still make Astarion or Tav a thief.
You could go Gloomstalker, any dex fighter fighter, or shadow monk.
Even if I use astarion hes probably getting rerolled into one of these.
