To want DH to maximise his own sleep

im realising I took so much huge mental load of everything in my marriage such that it drove me to actaul insanity

I think have a rota for mornings whatever feels a fair split. I totally get you want to do some mornings with other dc but I don't think you're maximising the opportunities to rest, it's inefficient for you to wake up, decide it's him, then try and get back to sleep whilst (I'm guessing) silently seething at him not approaching it the way you wish he would.
You definitely can't tell a grown man when to go to bed, he obviously gets something from his behaviour/bedtime and I get it, I'm a night owl and like to zone out whilst the house is quiet. I do however, neck a coffee and get on with packed lunches and school runs even if I've been up a lot.
Caring is very very hard, it sounds like you're both actually doing an amazing job of maintaining a good relationship and being a great team for your dc. Maybe also celebrate the fact this irritation is actually relatively minor/mild, that's an achievement. Then plan a morning rota, so he knows the days he'll be up early, and so do you. You'll need to accept that on those mornings there won't be huge energy, he'll do it differently, but he will do it. Take those mornings to rest and then you can approach your mornings your way, and your husband can catch up on sleep. The best advice I ever had, was if you delegate a job to someone, they will do it their way, so long as the end result is achieved you can't micromanage the approach. No, your husband doesn't infect the house with energy the way you would, but maybe, in his "I hate mornings" moments, your other dc will open up about things they hate and want to share, or, maybe, they'll choose to do something independent to help out - people do things differently, it feels wrong when you see something different to your style, but diversity is strength!

This sounds hard OP. But you both sound like a strong team, albeit there’s an imbalance at the moment.

from what I’m reading, you resent him not sleeping when he has the opportunity to. And you want your non-disabled sibling to have the attention from you that he/she deserves. You sleep and rest when you can.

i firmly believe that a fair and manageable routine helps family life no end. But it must also have flexibility. It seems as though his bed time and the family’s morning has no set routine so you are constantly uncertain of what the morning is going to look like.

can you suggest that you try a fixed morning routine for a while. If he does set days, you can wake and then have that glorious feeling of not having to get up straight away. And on ‘your’ days, you can plan a lovely morning with your DC which might include a special breakfast, walk to school, time spent plaiting their hair, stop off at bakery on way to school, lovely packed lunch, whatever floats your boat. But it’s more difficult to plan any of that if you don’t know if you’re on school run duty.

his decision whether or not to go to bed earlier. Wouldn’t it be nice if he came to bed with you from time to time? He could read, cuddle or sleep. Being together is so important.

If you're up late with one child then your husband should get up and sort out the other child and get them to school on his way to work in my opinion. Your sleep is important too. Totally not unreasonable. I'd be angry too

You do all the night wakes for one child. DH should be in charge of getting the other child to school. You need the rest. His sleep pattern is up to him so if he's tired it's up to him to sort that out.

Yes, sorry OP, I realise now you said you can carve some time out for yourself during the day and I leapt to the assumption about school. It is absolutely not unreasonable for you to expect your husband to think more about what the whole family needs and act accordingly but sadly at the moment that just doesn't seem to be his mindset. Until or unless that changes, you'll just need to be clear what you need from him as it sounds like he does respond to that. I totally get your frustration though and it is not unreasonable. It would be better if he could see that family resources (time, money, space etc) are shared and, when they are limited, then how each person uses those resources impacts on everyone else in the family but it sounds like he just doesn't see it that way.

"my time isn't my own in that they are usually with me and can only be left for a "pop to the shops" length of time."

Not at school, then. This is hugely relevant to your original question, because some people will assume that you get a break when your child is at school, but you don't.

This! Why are you deciding day by day, that's so chaotic. If you're up so much, he does everything in the morning. His decision when he goes to bed. Tell him that, and DO NOT GET UP IN THE MORNING. Or, if he doesn't like mornings (which is ridiculous when you have kids, noone likes mornings), he does his share in the late evening so you can get a block of sleep say 9pm-1am. Then you should feel more rested in the morning.

Can you steal 5 minutes with your husband on the sofa when it’s late. Would it help to just sit with each other and have a cuddle. Do you need to rush through to the next thing. It might make you feel less frustrated if there’s been some shared affection. Plus definitely schedule in at least one day a week where he always handles the morning stuff, that will give you a mental break on the organising everyone front.

What stands out to me here is you waking up in the morning and doing a quick evaluation of who’s the most tired and then that dictates who does what. You should agree who’s doing what in advance, and then if your DH has made himself tired by staying up that’s completely up to him and the only person affected is him.

A side point is also the unreasonable mental load on you of a tiredness assessment of 2 adults the moment you wake! Get rid of this awful way to start to the day.

Does you get DLA for your child? Does your child have an EHCP? Do they attend a childcare or school setting (mainstream or specialist)?

A PP mentioned school but I wonder if the child is not at school, given that you only get the opportunity to nap when your child naps.

I think your DH needs to deal with the mornings with your other DC as standard. You shouldn’t be waking up, calculating who is most tired, and asking him. It should be default that he just gets up and does it.

Sorry, OP, just seen you do get some 'me time' during the school day, which is definitely good. I think the rest of what I said still applies though.

I think the fundamental problem might be that you are looking at everything from a communal / partnership / whole family approach eg assessing who can get what sleep when, recognising that you each have a part to play in ensuring the other gets what they need, whereas he is looking at it from a more individual perspective ie I feel like chilling out and getting some down time to myself so that's what I'll do. He isn't joining the dots to see that by taking that downtime he is actually taking something from you, because he is then also tired and you then try / feel the need to help him with that. In a family with stretched resources (in this case, time / opportunity for sleep), anything one person takes does impact on the other family members, but I do think many people (men?) struggle with this idea. Not to be too Marxist about it but your approach is more 'From each according to ability, to each according to need' where he is thinking about himself and you as quite separate entities.

Ultimately I think the solution is as others have suggested, set the boundary of what he must do to support you and then it is his choice whether to stay up voluntarily making himself tired. I understand the frustration that he can't see for himself that his behaviour around sleep, and not taking the opportunity to maximise it for himself, does actually impact you, but he may get there in the end. In the meantime just don't allow yourself to feel guilty; if he knows he is getting up, it is entirely his choice to make himself tired. It may be the trade off is worth it to him to feel like he gets some 'me' time. That's fine, but it shouldn't negatively impact you when you're currently getting no 'me' time or proper sleep. And ideally he needs to better understand himself and his choices as part of a family unit, but that may take a bit longer.

edited to remove - realise you’ve already addressed this in your OP.

sorry you’re having a hard time at the moment

I understand that you feel that you want to help the other DC in the mornings to make it fair for him/her. However, I don’t understand why your husband doesn’t help at night with your SEND kid. I understand that he works but at least in our family even during mat leave my partner did nightshifts with the baby and I got to sleep and vice verca during his pat leave when I got back to work. Or maybe he can help you out on Friday/Sat to give you a break and time to watch a movie or just enjoy a glass of wine in peace. I am worried if one day you are going to have a mental breakdown with such disrupted/little sleep. Make sure you take care of yourself to also be there for your other DC. And no you are not YAIBU, you are exhausted!

Having to ask your dh to get up is enough to make anyone murderous. He should know. So he puts on his alarm, gets up , gets kids ready and goes. Stop being so nice saying he deserves his sleep etc. He can stay up all night if he wants but he must do the first shift in the morning otherwise you will collapse. So say nothing about him going to bed but lay it down that he is on morning duty. Its madness for a father responsible for children to stay up until 1.30 and then not jump out of bed and let his exhausted wife sleep. But let him cop on to that himself.
No wonder you are angry as underneath you know the level of unfairness is unreal.

"DH needs the unwinding time to be OK to carry on supporting the family"

what about you? Do you get "unwinding time"?