NC as will be outing but single working parent with 3 dc one with complex needs. Ex did EOW (moved away). Did very little holidays. so similar setup. I also know single parents with more one disabled child.
I managed but older dc would help alot as I did struggle with exhaustion. I had some respite. I have a parttime flexible wfh job and a very sympathetic manager.
You have no back up which can be scary - it’s almost certain there will be an emergency like a child needs to go a&E and you have to take everyone. Ideally you would need someone like a regular sitter or nanny or friend as a contingency plan. Your eldest should learn to drive.
worst is when you are sick yourself - it can then can become dangerous so he or family would need to step in.
I was able to take some time out during school/work days to go to an exercise class or meet people for coffee and this kept me sane. You need factor in some downtime and social time for you.
You should ask to be reassessed for more respite. You will get a different reaction as an effectively lone parent as the cost to the council if you burnout isn’t worth it. There will be specialist options which could include over night to give you a break.
I got a proper break eow when ex had dc as was then just me and I could sleep etc. my worry is you would not get a break even when he is back at weekends as if you are there he is going to rely on you and dc are going to be used to you/want you. I think you would need to consider going to a hotel when he is home and have a full childfree break and also him allowing you a holiday childfree (or you and your eldest).
school holidays will be a killer you will need extra respite then
Outsource what you can eg cleaner, he does all admin etc.
i would be worried your marriage won’t survive once he gets to live life without the restrictions of caring. It’s hard not to resent ex having a ‘normal’ life while mine is dominated by caring and I’m often on the brink of exhaustion. It’s like a switch flicked when we split up and he forgot what being a full time carer is like (or just feels too guilty he blocks it out). He has a new relationship, hobbies, social life etc and dc fit into his life on his terms. He doesn’t care I can do none of these things. Your dh would need to understand that when he’s back home you are 100% handing over the dc with complex needs to him. So in effect he would be working fulltime as a carer during his weekends ‘off’.
if you have bedroom space think about an au pair.