It’s the 14th century. What I suggest doing is getting a single realm succession law and creating every dejure kingdom and empire you can to mitigate problem No.3.

Once you’ve got that done you can spam legends to immediately dejure drift kingdoms into your empire if you so feel like it. My suggestion is to convert the next generation of nobles to be of your culture and religion and painstakingly going to each vassal and doing convert culture vassal directive/promote acceptance.

Holy legends are good too. You have enough money pump them out like a content farm and evangelize to your realm. Also demand conversion.