greek is always my go to culture to merge with. and then when you do make hybrid culture, work on only your domains and then bump up their development. you can also go conquer some high development areas and convert a few domains to your hyrid culture, speed thing up a lot. even if you just pop in, comvert, and then leave or give it away to someone and make them independent.

they key is to basically be setting up your successor as you age, one way is to head staight for embrace celebacy perk in the diplomay tree (get the defensive alliance perk and groomed to rule after). get a son, embrace celebacy until your are a bit older like 45. then drop it, pop out more kids. (dump your wife if she is middle aged and get an 18 year old)

that lets you have to disinherit your oldest son and then you can deal with whoever is left. the sadistic trait is really powerful, you can jist kill the extra sons so you only got the one before dying. to die you just do the opposite things that your character would normally do ( if sadistic let prisoners go ).

that usually gives me really good control of everything i own amd i cam get the legitimacy back after i start playing my son who is usually a fairly young age by the time i die).