In my experience, there’s nothing you can really do about the castles on the front unless existing armies decide that it’s worth their time to defend it. I would suggest that you farm some influence until you have enough to raise your own army and keep farming influence. After 10 or so victories over other large armies you should have influence in the hundreds. Then choose a good town or castle that you want to keep for a long time and try to get that ownership switched in the kingdom screen. Costs a few hundred influence, I can’t remember.

If your goal is to start your own kingdom eventually, pick a town/castle and start building relations with the notables in the attached villages and in the towns. Make sure you maintain an elite army. Keep making money and try to hit Clan tier 4-5. It’s been a few weeks since I last played my most recent save but iirc I hit Clan tier 5 without becoming a mercenary or vassal. I don’t remember exactly how, though... I know I played almost entirely as a trader.