In all honesty, this man sounds like he has checked out quite literally.
He isn’t committed to you or the kids. He views them as your responsibility. You shouldn’t have gone part time when he already earns x3. You also shouldn’t be paying 50/50.
Regardless of the divorce (I doubt he would mind), he already earns x3 and you both sound quite young. Eventually, he will swan off to the sunset and earn x4 and x5 more than you.
Women shouldn’t shoulder the whole burden of caring for kids. You shouldn’t have had the second child when you know the first has complex needs. You’re going to be on benefits, meanwhile he won’t struggle one bit. He can easily get another mortgage. I’m not saying to be harsh, but he will most likely start a new family as he is relatively young and just CMS to the ones he already has.
Unfortunately, you ruined your own future and children’s by taking the whole responsibility of the kids. Not expecting him to do it and share 50/50. You need to use your backbone, say no and be strong. You ruined your career to preserve his. Now, you want to be dependent on him but he won’t support you.
There’s still time to change this. Demand 50/50, go back full time and make him go to appointments. If he had to reduce his hours, that’s his problem.
Do you have a pension?