Cleared chapter 6, and I can now confirm thieves are straight busted. I stole Morholt's Waterwheel and other spells, then farmed infinite hits off of him with low damage weapons.
You can also strip an enemy of weapons, body block them, and then endlessly steal a vulnerary off and onto them for massive exp.
Also, I did some more thinking, and I know that crits are part of the FE formula, but what if you just removed enemies' ability to crit? Crits are interesting on the player's side, and I don't think they're 100% a bad mechanic, but on the computer's side they're sort of just an RNG check to see whether you need to redo a map. They might not be essential to the game's balance.