Are all your children at school? Because if that's the case, surely you get quite a few hours for yourself. I understand they have additional needs but it can't be every single day.
This is spectacularly ignorant. SN kids often can't be worked around when they're home, they often need full attention. So the only time to do the mountains of admin that comes with having SN kids, as well as the usual life admin, and all the housework, food shopping, running errands etc, is when the kids aren't there. Meaning if she starts taking time for herself during school hours, chances are important things just won't get done and family life (read OP's life) will get harder as a result. Or they will get done, but only because she runs herself ragged another day working at double speed. Knowing this will be the result of taking 'time off' negates any relaxing qualities of it.
I am usually upstairs with dd until she's asleep which can be 9-10pm, come down and make lunches for the next day
And what is husband doing whilst you do this? Let me guess - whatever he likes. He should be doing either bedtime or housework, lunches etc.
He does take the kids out at the weekend to the park or something.
There you are, talking like he's doing you a favour... And what are you doing while he's taking the kids out? Housework, I'll bet.
You should be putting your feet up and both doing the housework when he returns home. If you did start expecting his help on return, I'll bet he stops taking them out. Proving he sees parenting the kids as 'work' when it's him doing it, equivalent to you doing housework. But when you're parenting the kids it's nothing, definitely not equal to him being at work. Funny that.
He's treating you like you're staff.
You may want to 'work things out' and not LTB, but you can't make a relationship work all by yourself, it takes two. At the moment he's treating your family life as something he can drop into and out of as he chooses, an optional extra in his life, instead of seeing it as his duty to contribute practically as well as financially. OP, you ever hear the phrase "don't prioritise someone who treats you as an option"?
If you want to stay together, one last chance for him to step up and treat you as an equal. Or else force some semblance of equality on him (it won't be truly equal because you'll still end up doing majority evenings/weekends/nights etc) by returning to work and he'll have to pay a fair amount towards the full time nanny or whatever is needed to make that situation work, with both of you having equal spends after bills/nanny/children related expenses etc are paid.
If it's not possible for you jointly to earn enough to pay the bills and the nanny, then he needs to recognise the huge huge sacrifice you're making by giving up your career to be a SAHM, be grateful and treat you with the proper respect for it.