Love the game, been playing Ck2 and 3 for about a decade now.
A big part is making sure your lands are not split up too much each generation. Usually this means conquering or revoking from a vassal a new duchy per son, so the core remains intact on sucession. Like real life, Tyranny is just a number if you have the army to back it up.
Another big part is military strength. Betroth your newborns to people with big armies. Keep in mind the product of said unions will inherit claims to your land. Also, Knights and Men at Arms are wayyyy stronger than levies. If you’re playing Norse, they cost prestige, not gold, so you can maintain as many as your monthly prestige income allows, and battle, feast, and raid to boost your prestige over time to afford more.
Raiding is a HUGE boon for the Norse. Best places are counties in Ireland early, and then Brittany, Spain, and Cornwall as you become more powerful. Just be sure you scout the place out and make sure the ruler doesn’t have better troops than you, or you’ll go home with nothing!