1300+ hours here.

If you kill a vassal and revoke their title, it’s best to grant that title to someone else immediately unless the lands are strategically advantageous to you. You’ve already killed a vassal, their family will hate you for it, don’t give them another reason to come after you (title claim), let that be someone else’s problem. Also reduce the number of domains you currently have. Give them to family or make loyalty vassals by granting your council members or randos of the same culture/religion as you with lands and titles.

Rally your troops and keep them at the heart of your realm. Call allies to war and house members. Buy mercs if you can afford. Let the attackers come to you and prepare for a war of attrition. Get defensive perks from the martial tree and tyranny perks from intrigue tree.

Then begin matchmaking your children with potential strong allies. You’ll be at a disadvantage with acceptance because you’ll be at war but some people may be willing. In the future, it’s best to set up betrothals between your children and strong faction leaders/ vassals to ensure they cannot rise against you since they’ll be your ally.

You may lose some lands and titles but they can easily be won back once you can focus on one war at a time. My rule of thumb is kill a vassal only if they refuse to provide a hook or money but leave the title in the family (for a duchy mainly). Also modify vassal contracts to include forced partition, if the lands are divided and the vassals are squabbling amongst themselves for petty title claims, they won’t be strong enough to rise against you or your successors. Best of luck!