Tips for starting with Carantania in 769 AD?

[bati]

Hey guys, new player here.

I've been trying to get into the game for the past few days, trying out various starts and I eventually settled on the noob island, Ireland 1066 AD, as per many recommendations, but I'm having huge issues with their names, to the point that I have to double or triple check which character I'm looking at.

So I figured I'd try looking for some rules in my home region and I found High Chief Valhun of Carantania - my preferred option would be Chief Hraber of Stajerska but that's count level and from what I read duke with no direct liege is the optimal way to learn the game.

Anyway, Valhun is Catholic so I'm not really sure how to start things - should I convert to Slavic religion or start converting my realm to Catholicism? My vassals seem quite bloodthirsty, expanding to the east towards Croatia but from what I can tell I don't have any CBs there, only on Istria which is part of Kingdom of Lombardia (?) and that means huge resistance.

Any and all tips will be appreciated, thank you.

edit: Forgot to mention, I own all the DLCs, so seem to have a lot of options available.