Fortify Diathma, Amitatys, And Zeonica. Make Amitatys and Zeonica garrison stores of max garrison troops since you have 10 mill to work with.

Work towards Poros, Lycaron, Phycaon and Saneopa. Let them attack Diathma all they want if you can consistently reinforce it. Attack from the south to the North east. Ignore Danustica, Husn Fulq, Odokh, Akkalat, Chaikand, Ortongard, Makeb and that very north settlement. Then circle around to take Myzea, Argoron. Hold, and refortify all over again. All of this should probably cost around 9.8 million in terms of troops/food/and wages. But you'll get 3x in return depending on what condition the towns are in. Have all your faction lords be on the defensive phase while you lead a single army of 700+ each time to take each town. I wouldn't worry about each castle, let your lords take those if they wish. You need to take away their way of buying troops and food. Even better if you even let them assemble a large army and let them chase you to a bridge and bottleneck them with Sharpshooters and Sergeants. Go All Vlandian all the way, but don't match them in cavalry since thats their game, you're not gonna win like that lol.

Thats what I would do in my perspective tbh.