Steppe bandit tree eventually upgrades to Khan's guard tree. In my Khuzait games I used to always clear steppe bandit hideouts with tournament arrows and have an army of only Steppe raiders that I could then upgrade to the Khan's guard.
I have to admit that I only reached disciplinarian perk when using the export character mod cheat but that is because of my playstyle, if you don't join a faction as a noble you can't form armies and thus can only level leadership from having 75 morale at least and that is super slow.
Do bandit troops still transform into their noble counterparts when garrisoned in castles?
if not, time to sell my prisoners and place all my bandit troops in a specific group and have them all suicide charge, at least I'll level medicine a bit that way