I honestly think he has mental health issues he only sees things from his view
Just as you having depression doesn't make you an unfit mother, him having any kind of mental health problems (even if so) doesn't excuse him. He's an abuser and an asshole: that's what you need to focus on at the moment.
I know many people with mental health problems who would never treat a loved one or indeed anybody) like this.
Keep documenting things. Talk to your GP/HV. I believe, given what you describe, the police would be interested, as well (well, the switched-on forces who are aware that coercive control is now illegal) - you've already mentioned isolation and attempts coercive sex, blackmail, verbal abuse, rages, and that that's only the tip of the iceberg. It may well be pushing the limits of your bravery, but it will protect you when, maybe not even far down the line, he really does try to keep the children.
A lot of abusers threaten this and don't mean it - they just want to control and blackmail the abuser into compliance, when in reality they want nothing to do with the actual reality of raising children
But far - far - too many abusers will take the threat and the proceedings as far as they can go, and sorry but yours sounds like that kind of guy. The more evidence you have of his actions, the more outside support you can get, the less likely he'll succeed.
Get strong. Stay strong.