How can one deal with tribal, pagan vassals

I started my first game with the Charlemagne DLC in 796. I am playing as a Frankish Count and trying to suck up to the King so he would grant me a Duchy. It sort of worked, but not until after he won a Holy War on the Saxon pagans. He granted me a Duchy in Saxony with 3 pagan, tribal vassals.

20 years later and three revolts later, I've managed to convert all 3 provinces to 'majority' catholic and replace one pagan with a Catholic sycophant vassal. However, his holding is terrible and I'm strapped for cash having to call levies out all the time to fight revolts to a stalemate – they have dynastic and marital allies, but I have none I can call and can't call any religious cohorts as it's an uprising and not a holy war. The other two uprisings I've had to end by assassination of the leaders. This could gradually end my pagan problem, but I don't think I'll be able to keep pace.

I can't demand conversion directly as they hate me, and they hate me because they won't convert. I can't leverage my catholic vassal in pagan lands because he has 'wrong holding type' penalties on taxes and levies.

What can I do to turn this burden into a blessing and continue conquering the world?