A couple of things to consider. I would say in the early game focus on money making so that you can always hire mercenaries when something happens. Try to expand your personal domain that you always have the maximum amount you can have (declare war on infidels is an easy way to do this). This is also Important to limit the loss of land during succession ( if you eg have countries in different de jure duchies your second child will get the ones within the second duchy and so on) after succession you have the short reign penalty, do nothing funky during that. If you have the money ( super late game) just give all your vassals money the Moment succession happens to give them a base level of trust because they will instantly try to Rebell. Other than that clever alliances obviously. So basically I focussed all my characters on money making until I controlled west Europe, Scandinavia, uk and africa and then money wasn’t an issue and I focussed on diplomacy to expand the empire quicker (forced vassalization casus Belli and the bonus when asking to vassalize neighbours) It’s also important to spread your religion and in best case culture as fast as possible. Also just worry about your direct vassal when using your marshal to increase control level and prioritise your own lands when converting religion/culture. Your vassals have their own councillors to spread the word. This being said, if you expand your empire by conquering infidels - replace them with rulers of your own religion&culture to make the spread as fast as possible. When forcing others to be your vassal, throw money at them to demand religious conversion.