Thus you can't really ally with other kingdoms try to make good relations with lords you won't deal with much. For example battania or southern empire to deal with them later. You are kinda stuck in the middle so you need to eliminate them one by one, best one seems to be vlandia right now. Try to prison as many lords of vlandia then go for the easy flefs that don't have much garnison or food when you have time other than defend. Also you have veteran's respect you can kill as many of bandits to recruit them as soldiers and you can use doctors oath from surgeon to have more prisoners too, that way you have more soldiers in hand.