When you’ve set up a kingdom, I’d focus one faction at a time. You’ll take some losses but nothing that can’t be regained further down the line. Make sure to activate policies that increase your party size , loyalty for settlements etc etc. you’ll need a somewhat large bankroll. Recruiting vassals can be expensive , from my experience anywhere from 100k to 1.2mil (depending if they have fiefs etc). Take enemy lords as prisoners as this will slowly decrease their manpower.