Attack a weak castle, if you got it yourself as a vassal you can take out the defenders, except the natural garrison. Speed attack it with a big army. If you clan tier is at least 4 you should be able to take either one of your own or a recently captured one. Once you have it, immediately form your own kingdom. Use your family members for buffer armies and unless its in a good position, abandon the castle. Now with a 600+ army with your family members, you should be able to defeat just about any castle given it haven't had unnatural high growth. And build from there. Declare peace if you must and take time to build up your influence. Pick Kingdom perks to help you like Royal guard +40 or 60 Party size for The king (You) And + 15 members for clan leaders (ALso you) And you can easily lead a 300 man squad. You may have to have some big coffers before this but, once you do, you can still very much form your own kingdom from scratch.
With a new personal perk, you can have 4 extra armies in your clan, +1 from the three earlier meaning your brother all siblings and your wife can lead armies.
If you still struggle you can always reduce difficulty if you aren't already playing on a easier one. On the easiest difficulty your army, unless they glitch heavily should be able to smash just about any other army easily.
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