Strategical peace to shore up defences / focus on one side of the map.
Set your kingdom where you can defend it (I like edges of the map and near long seas)
Waging war on enemy territory means you weaken them even if you don't hold territory.
Attacking cities/castles of enemy clan will help you keep relationship high.
You NEED to pull enemy clans to your side.
Best keep your companions in own army, because AI throws away elite units with no care.
Pull big armies and disband them to cause your lords to recruit where (and what) you want them to recruit.
Counter attack enemy armies when they've bled on walls of your city/castle.
Set traps with low count high level garrisons. AI beelines for garrisons with low number of units even if it's Palatinates, Fians and Darkhans.
Don't overextend, and supply own cities with food, but also control garrisons, and use their recruitment, especially to pull cavalry.
Winning as own kingdom is just a matter of strategy, might not work on first few tries, but that doesn't make it "pointless".
tl:dr - git gud.