Glad to hear you’re enjoying it. I have to admit I think CK2 is still superior (as it has more storyline material), but the mechanics in CK3 are better.
As Norse, you absolutely need to use your own raiders to collect money and prestige, then use the gained prestige to build up your hird of professional warriors (you pay Prestige for them, rather than money). Rinse and repeat to build up a big “treasury” and a large force. If enemies raid you, raise up the hird and crush them. If you are out raiding at the time (which is when they’ll usually come knocking), raid them back later (Norse don’t turn the other cheek).
Alliances are tricky. You want to be allied with characters who can support you in wars, but who won’t drag you into a lot of losing wars. Ideally, you want to be allied to someone who is the top dog in their region, which will usually mean they are strong enough to help you win your wars, and that they can win wars without your help. That way you can contribute to their wars and get prestige cheaply (ideally, they defeat the enemy army, and then you can send in a small force to help them siege), while also earning their favor for helping them.
Generally, I do four things with marriages:
- Secure the loyalty of my vassals. A vassal who is allied to you through marriage cannot revolt against you, so this is a great way to secure the realm with a new ruler.
- Alliances with powerful rulers who can be of help in war. Most useful in the early game, and - as you’ve found - can be a double-edged sword. Note also that if you join a war but don’t live up to your commitments, they are liable to call you out about it.
- Dynastic marriages that allow your dynasty to take over new lands. Can be difficult, but important for getting renown.
- Improving the bloodline. Probably the majority of marriages, when I play (I’m apparently a Bene Gesserit). But it’s a smart thing to do even if you don’t want to go too much down that road, as you get get sons-in-laws who can be used to fill court positions and fight as champions (with good chances that the following generation may also contain useful cousins of your dynasty).