So, the game is wonky but empire building is possible. Though in this play through I think I've hit the peak of my arc for awhile. I started in the 960 years, and being a tribe is very tough to build up a large empire. Being tribal and Asatru religion makes it so you can't really upgrade your government to a feudal system, so your succession laws are locked into "Partition" or "Elective." I got lucky and my one ruler died from being gored by a boar during a hunt and my son was able to take over from 16y/o giving me the maximum time to do stuff with a single solid ruler.
However, he is getting older and he is going to die. I'm currently the King of Iceland (player created kingdom) Ireland, Wales, and Sweden. When I die its all going to go to election, and while I will most likely keep Iceland and Ireland, Sweden and Wales are prolly going to go to other people cause of the vote system. This happened prior with Wales, but I was able to bring it back under my control via a little murder. Oh well.
Big empires though get extremely unwieldy unless you have the government to support it. And starting as one of the non Catholic realms so far seems to put you at a significant disadvantage cause you have a Tribal government structure and an unreformed religion. So you have to reform your religion and develop your culture to the point where you can enter the feudal government system and hopefully have more control over your realms and inheritance. I haven't played as a Feudal land yet. Too much fun fucking off in minor countries trying to do dumb stuff.
The other thing I think I would do on another playthrough is purpose build my leader towards an objective. With the RPG talents system you can do some dumb shit. But, if I was to do it again (and will on next playthrough) I'd go for a single main objective. "Reform the religion" with this ruler, and go all into learning and religion to lower the costs. Next ruler maybe have him geared all into the martial traits to expand the kingdom. Then next one go deep into prestige so you can form an empire etc. You don't have to stick to one tree for your lifetime, or even one 'path' in each lifestyle type. You can mix and match, and getting to know the game a little better a lot of the skill point trees seem to be traps, or are only good for one very specific thing. Look over them carefully. You don't have to buy all the way to the end to start another.
So anyway, in this play through though I think I've peaked for awhile. I'm going to get hit by the inheritance tax and lose half my kingdom when I die,, and I've just managed to reform Asatru as a religion with myself as the religious leader, but the Pope just declared a holy war to reclaim England from the pagans. So its about ti get hairy. If I survive though I think my next goal will be to destroy the Papacy. Fuck you for your holy war.
But yeah. Half the game is just how fucked up shit can go, and laughing at it while struggling on despite it.