You poor thing. No wonder you’re shattered 💐. You’re effectively a lone parent. That’s tough. First thing to say is it gets a lot easier as the children grow and become more independent.
Why did your DH have children? For a woman to look after? He needs to realise that if you want to be a parent, you’re responsible for looking after the child at least 50% of the time, including overnight: 24/7. That’s the default starting point. If you don’t want to do that job 50% of the time, don’t have kids.
Of course, parents can negotiate to suit their needs, but that’s a negotiation. Not to be assumed by the father. For example, if you worked part-time but could still do your 50% of every 24 hours, but he couldn’t, he’s responsible for organising and paying for childcare for his 50%. Not you.
That costs money (childminder, nanny etc) because, guess what, looking after children is a job. A very responsible, important, challenging, tiring job. Caring for children with SEND is a far more specialist and demanding job. So, you have a specialist, demanding job, which you appear to work at around 14hrs a day, 7 days a week. He needs to pay you for it. The going rate, with paid leave.
Household chores (cooking, cleaning, clearing out, life admin) should also be shared 50/50. Or he has to pay someone, e.g. a cleaner, to do his share. Or (as with childcare) pay you to do it. The going rate.
Your DH needs to start taking his parental responsibilities seriously. You can’t be a good parent and not be there most if the time. He needs to change job, so he can cover his 50% or much closer to it. He needs to stop treating you as an unappreciated servant rather than a paid specialist childcarer. Does he love you and the kids? It doesn’t sound like it.
If he can’t change his perspective and take more responsibility, I suggest you consider why you’re in a relationship with him. If it’s only for financial reasons, then you should start considering your options for being financially independent. If you split, he’ll either be responsible for 50% of the childcare or, if you remain the main carer, will have pay you for it.
In the meantime, accept the help you’re being offered. Your kids (other than perhaps your disabled child) should be fine if looked after by friends or a good paid childcarer. They’re used to being away from you at school. I know the problem with the nursery worker must have affected you, but most childcare professionals are fantastic and you can ask for recommendations. Parenting is all about encouraging your kids to gradually become independent from you, and you re-emerging into your own, independent life. So, don’t give up your voluntary work if you enjoy it and find it rewarding. It gives you a role outside of childcare and housework and may be v helpful in getting a career going again.
Housework should be possible when your children are at school. I have three DC and was a lone parent. I work full-time and manage to do housework in the evenings and weekends. You learn to be really efficient, e.g. cleaning the bathroom while they’re in the bath, batch cooking etc. But, given how exhausted you are, you may need a hand declutterring and getting systems in place. It can help to break it down into different chores for different days of the week or month. The kids should all have their set chores too: laying the table, washing up, sorting the laundry, tidying their rooms etc.