You are NOT the one doing wrong!!!!!
I don't know the law in the states but I don't see how he can have visitation rights if he moves out of state unless he comes back and sees them in your state.
They are too young to travel on their own and he would have to get a permanent housing to be able to have kids there.
Plus since he is starting his own business that is not established yet he wouldn't have a chance in court.
But don't tell him this. See a lawyer and let the lawyer handle it with him.
And don't let him bully you into having so everything "signed and ready for him".
Document what you can including any emails/texts he sends you.
You are the stable presence for your kids. Let him shoot himself in the foot. You stay calm and legal.
You are a good mom and person. ((((((FaithnMe)))))))
You are hurting right now. When you realize what he is doing you will feel more like standing up for yourself and think F him. I deserve better treatment than this.