This sounds similar to my DD at the same age, and we were floundering hugely. School also reported no concerns as she masked so effectively . School wouldn’t refer for Ed Psych ax and in our area, the ND clinic will only accept referrals from the education setting.
I am going to list what I think worked for us, as 5 years on she is a different child.
We read a great deal about ASD, ADHD and PDA and decided to adopt low demand parenting - we did lots of research and agreed that we would both be consistent.
We explained to DD and DS that we would be making changes to our parenting because we wanted her , and our family to be happier. On the whole, we adopted the same approach with him.
We stopped using consequences, other than natural ones. Often this was reframing things. So we may still leave somewhere early if her behaviour was awful , but rather than saying it in that way, we would say that she was obv struggling and we were going to take her home to keep her safe and allow her to have some quiet time.
No reward systems.
Pictorial daily timetable. More day to day routine and stability.
We used time in instead of time out.
When she became very violent / affecting DS, I would carry her to her room (or both together if needed) - but not angry, calm and soothing . I would go in with her and sit in front of the door blocking it shut so she could not leave and hurt her brother or herself. I would tell her that I could not leave her until I knew she was safe. Being held very tightly sometimes helped. If she was violent to me, I would try to hold her or I would fold up, head tucked into knees etc. I didn’t engage verbally if she was screaming at me other than to repeat a calm platitude.
We stopped a great deal of activities / social events etc and had a calmer and quieter family life. Lots of walks in nature one to one.
We allowed her to miss school at times.
Covid showed us how much school was affecting her and things improved loads. When school reopened , she returned for a while but when the school still refused to refer her, we removed her at the end of Y4 and home edded her for 18 months, finding lots of nature and animal petting type things. We found the majority of the local home Ed community were ND children failed by the system.
During this time , we had her privately assessed and diagnosed with both ASD and ADHD. We then lobbied the LEA, local schools etc and she is now statemented and has loads of support. She returned to a different school for the last term of Y6 and is now thriving in Y8.
Our approach to parenting has completely changed and our household is so much happier. She has also naturally matured and is gradually learning to articulate her needs - eg, if we are out somewhere - restaurant , panto etc and she becomes overwhelmed/ stimulated, she tells us and we just leave with her , no arguments.
Sensory things , she has a swing sensory hammock thing. She also has a pull up bar in her room to swing on, and a big yoga ball. Her room is her safe haven and we never turf her out for visitors as we once did.
Often, she cannot recognise hunger or thirst - common with ND. And if you offered food /drink / made food suggestions or asked her if she was hungry, she would kick off. We started just making her a healthy snack and drink that we knew she likes, and delivering it to her -
allowing her to eat in her room if need be. She would say she didn’t want it and I’d just say ok, I’ll leave it there just in case you change your mind - it was always eaten and the hangriness would go.
lower expectations- most ND children operate emotional a couple of years younger than their chronological years.
one good example of a simple parenting change - asking her to have a bath would result in screaming / shouting / sometimes hitting and refusal.
Suggesting a bath resulted in her being stroppy, oppositional and refusing.
What worked - running her a lovely deep bubble bath with low lights . And saying to her casually/ in passing that there is a bath ready if she was interested, and walk away without asking her if she would have it. And it worked. Now, she runs baths like this herself and tells us how much they help to calm her down.
Finally, loads of love, affection, warmth. Take her lead if she doesn’t want to be hugged don’t force it but find other ways to be close - walks , reading to her etc.
it is so hard because of course with all of this, you also have to make time for your other DC as there needs will also be higher because of her. We talk to DS about it when she isnt around and give him an outlet to express the impact on him. And we have ended up do lots of things with one parent one child split to benefit them both. The need for this now is lessening.
I hope that some of that has been helpful. You are in the thick of it, be kind to yourself.