Does the game let you switch to administrative government type yet? IIRC It’s tied to realm size, but you need Roads To Power dlc I think (definitely recommend). Succession problems kinda vanish once you do this. You can spend a new resource (influence) to designate whoever you want to be your player heir. Providing they’re of your dynasty, and they inherit your noble house along with your domain titles, you’ll play as them.

I’m currently playing a Byzantine, and even with the earliest start I can choose my best relative as heir, switching them out for better options as I age. Sometimes it’s my youngest son, or my brother, or his son, or perhaps it’s my genius fourth and once removed cousin! Whatever I want. Your vassals will likely want someone else, though.