I know you wrote this post a while ago OP, but I've just happened upon it.
My on DS is autistic and started September 2023.
After his place was confirmed I called ahead and spoke to the SENCO who was an absolute angel. Truly a godsend for my son at that school, but I think it was important for me to make first contact.
She explained the staff to children ratios and we talked through the EHCP process and what short and long term outcomes we were looking for, and what obstacles we faced.
My son has no consistent motivators and is very self directed and cannot reliably follow adult instructions which were important to recognise. He also could not wait or sit still and so going on schedule did not work either.
This meant we could get my son as close to a 121 as possible, and also agree on an intimate care plan.
The school also put us in touch with the school health visitor, who wasn't a massive help to be honest but it meant at least some medical professional was documenting our struggles.
We have done so many webinars on potty training my head is in a wizz and none of it has helped.
We have been consistent about proper pants at school and sent many spares.
But at 5, he is now just starting to get it. Still has accidents and also withholds sometimes, but this time last year I didn't think we'd be close to toilet training until closer to 10!
Mumsnet is an incredibly ableist place sometimes so please don't let it get you down and I hope this helps.