I am really sorry you are experiencing this. It's very difficult to arrange child contact with someone who works shifts. It means there's a constant source of conflict and you feel like they constantly have the upper hand. That feeling of powerlessness is awful.
The system is really unfair. You can't make him do anything. You can't make him be reasonable. I think accepting this is really important. If you keep fighting to change it, you will hurt yourself more and you won't achieve it. It's like you're fighting a brick wall and damaging your hands. Someone should make him step up and take responsibility. You aren't wrong to think he should be forced to do his part, but it won't happen.
If he's got a changing shift pattern you are unlikely to get a helpful child arrangements order, unless he has control over his working pattern. And even then there's nothing to stop him cancelling at the last minute. It may be worth getting legal advice to see what may be achievable.
It's worth going through CMA for maintenance. It takes away control for him and reduces the arguments and the sense that he's witholding information from you.
In my experience, respite care for children with disabilities is almost impossible to get. You may get help from social services but my experience is that people assume there is far more help available than there actually is. It is worth exploring though, as the amount of support available will vary from council to council.
It isn't fair that you don't have time for a relationship, or to yourself. It isn't fair. It's understandable that you are angry about it. And you probably don't have time or energy to do all the things which might make things easier, or to develop the support networks you so desperately need.
And your ex is completely getting away with it. He's not doing the hard work, he has it all his own way, he's lying to himself and everyone else about how he's a great dad. You love your children but chances are the majority of your time with them isn't 'rewarding' - it's drudgery. He's completely oblivious to the fact that he is a terrible person behaving badly. And he's making your already-impossible life harder. This is also unfair.
However, this is the situation you are in. Working out how to live with the life you have will be more beneficial in the long run. You are already living in this situation and wanting it to be different isn't serving you. Focus on what you have agency over. You are living your life with your children - make it work with the resources that you have.