You stated he is a good dad. Then I don’t see why it would be an issue if you split and share custody. 50/50 or 60/40 or whatever. I misunderstood that you are afraid he’ll have access to the kids alone without you and he’ll mistreat the children as he is not a good dad hence you cannot leave him (understandable).

If he is a good dad, then separating/breaking up and co parenting like other divorced couples should work ok.