I had the same problem. In this patch the AI tends to declare war on the weakest factions, which means they gang up on you as soon as you declare a new kingdom. I restarted my play through and built relations with 2 clans by being a vassal in the same kingdom as them and voting for them to get all the fiefs I didn't want. I built the relationship to 100 with both clans I wanted to recruit because creating a kingdom and keeping your fiefs is a -40 hit to relations with every clan in that kingdom. That's unavoidable. If I were you I would make sure you have more territory than just one castle before you create a kingdom. I had 4 castles and one city. The 2 vassals I recruited each had a city and a few castles as well.
Then when you're ready to create a kingdom, create an army with the head of the clans you want to recruit in the army. Go to some place where it makes it hard for them to run away from you(a natural dead end). Disband your army, create your kingdom, and recruit the clan leaders. Save before you do this because the clan leaders might form an army as soon as you create a kingdom. You can't recruit them if they're in an army. Then save scum the dogshi+e out of your recruiting attempts until you get it right. It might help to have high charm before you attempt this. I did. I don't know if it's necessary. I recruited one clan easy enough but the second one was already in an army before I got to him so I killed him off then went to his home city and waited outside until he reappeared. Then I recruited him. If you recruit vassals that you're at war with they bring their fiefs with them so don't make peace until you recruit the lords you want. When you do create your kingdom the clans you bring with you will have 60 relations and property so they WONT leave you as long as you don't lose all their fiefs.
It also helps if the faction you're leaving is at war with someone when you leave them so they cant focus solely on you.
When I finished the process I had most of Battanias mountain top territories(Seonan, Dunglanys, Pen Cannoc, and about 6 castles) to start. I wasn't the biggest pu$$y on earth so not everyone declared war on me right away. I still had to save scum to keep Vlandia from wiping me out since they were massive. I would load a save if they did declare on me. By taking a lot of Battanias territory with me when I started, it made them almost weaker than me to start, so when I took Marunath it flipped my power and theirs. Then Vlandia declared war on Battania instead of me, which allowed me to take as many Battanian territories that I didn't have before Vlandia gobbled them all up.
It helps if you keep the faction your joining weak before you declare your own kingdom. I played early game as a mercenary for whomever was at war with Battania because I knew I was planning on joining them and leaving them. That way when you create your own kingdom, they aren't that strong to start because they will declare war on you.
Yes, it's kind of gamey and it involves save scumming some things, but it worked. Now I'm well established and the game is fun.
I will give it a try.
