I am naturally laissez faire nd relaxed, but my children ( same SEN as you) crave structure, rules and clear boundaries.
Boring! But it works.
They are allowed 30 mins screen time in the morning and one hour in the aftenoon. I give " 5 minutes to end" warning.
When they complained that they had only had 5 minutes, not half an hour, I started using the egg timer. That felt fair to them. Still use it at times.
Bed time and bed time routine set in stone, pretty much, but always room for a chat and cuddle and a bit more of a cuddle and chat.
Also do thinks as unhurried as possible ( less stress) and with advance warning ( we are leaving in 10 minutes, please get your shoes on, etc).
I don't give them a lot of choice, or negotiations about stuff. I mostly tell them what we are going to do, eat, watch. I find telling, not asking to work well. If they have suggestions, I listen, I am not some sort of dictator, and I might say it is ok ( if it is ) or just" good idea but we are not doing that this time"
I am not saying my way is the only way, just that I figured out being a slightlymore firm parent than I would be naturally works for them.
They know where they stand and they like that.
We don't do endless negotiations, but if they are upset I will sit down and listen and say that I understand that they are upset, but that what they want just is not possible right now.