Oh lovely, I really feel for you. This sounds absolutely exhausting, and it's clear how much you are trying to hold together for everyone in the family.
One thing that occurred to me is whether there could be some hormonal changes going on as well at 11. I noticed quite a big change in my daughter around that age, particularly increased anxiety, difficulty getting to sleep and generally much bigger emotions. Obviously that may only be one small piece of what is going on, particularly with suspected autism and ADHD, but if the sleep problems and heightened anxiety are relatively recent it might be worth keeping in mind.
We also have an autistic son, so we are very familiar with meltdowns and some PDA-type behaviour. A few things have helped in our family, although obviously every child is completely different.
Equine therapy has been amazing for our son. I appreciate it is not accessible to everyone and very much depends on where you live, but it seems to regulate him in a way very few other things do and he gets an enormous amount from it.
My daughter is neurotypical but has very high anxiety, and art therapy has been brilliant for her. I don't know if you have ever explored anything like that, but it can be such a gentle way for children to process things they may not be able to explain properly in words.
The sleep piece really jumped out at me in your post because lack of sleep can make absolutely everything harder, for her and for all of you. I wonder if, at a time when she is relatively calm, you could involve her in designing an evening routine rather than trying to impose one during the difficult part of the evening. Almost, "We know evenings are really hard at the moment. What could we put in place together that might make them feel easier?"
My daughter was really struggling to get to sleep for a while, and we eventually agreed on quite a structured routine. She is 10. At 7 she goes upstairs, gets into her pyjamas and starts winding down. I spend some time with her and read with her. From about 7.30 to 8 she reads by herself, then from 8 to 9 she can do art, journalling or creative writing at her desk. After that she comes downstairs for some milk and then goes up to bed. We actually made a nice little poster of the routine together and put it beside her bed, which helped make it feel like something that belonged to her rather than a set of rules being imposed on her.
We also bought her a CalmCarry. I was deeply sceptical about it and am still not entirely sure how much of the effect is physiological and how much is simply that she believes it helps, but honestly I don't care because she loves it and finds it soothing! A friend whose daughter was struggling with sleep during a very stressful family period recommended it to me.
We also got one of those bedtime lights that gradually dims and plays calming sounds. Ours happens to be a Hatch Restore 2, but there are lots of different versions and models available, so definitely have a look online rather than taking that as a specific recommendation. She really likes having the light and sounds as part of her bedtime routine. None of these things was an instant fix, but together they have made evenings much calmer.
And please don't lose sight of how much pressure you are under too. What you are describing would wear anybody down. You clearly love both of your children enormously and are desperately trying to protect them both.
The one thing I would say is that, even if you are fairly sure the comments about wanting to die only happen during meltdowns, I would still tell her GP or whoever is supporting her exactly what she is saying. It may be her way of expressing completely overwhelming distress rather than an intention to harm herself, but it deserves to be taken seriously and you shouldn't have to carry the worry about it alone. If you ever think she may actually be in immediate danger, seek urgent help.
Keep posting. There are lots of us going through this reality at different times and in different ways, even though it can feel horribly lonely when you are in the middle of it. I really hope you get some proper support around all of you soon. Sending lots of support.