I'm replying to your last message without using the "top-down" convention to put the hopefully-useful things first.
Of course I would ask her those questions if she would understand and answer me.
Would she understand questions along the lines of: Does the ponytail hurt your head? Is it heavy? Would you like to try two "bunches" instead?
Cutting her hair is an huge change for her
Is it the change to her hair itself that she wouldn't like or is there a salon trip that she doesn't like? [ETA: it might not be clear that my point is that if she rejects the salon but not the shorter style, it might be worth you cutting her hair at home.] My mum cut my hair at that age because I did not cope well with the one salon trip she tried. For reasons I'll go into further down, I spent most of my primary years with my hair between jaw and collarbone with one of those soft cotton all-the-way-around Alice bands (not the plastic horseshoe type) to keep it out of my face. It was too short to tangle much and didn't need a hairdryer to dry. I don't know whether the bob cut with Alice band approach might help DD?
I’d love to know why you think I wouldn’t do all these basic things? Or why I would be keeping it long for any reasons than thinking purely of her?
Two reasons.
First reason: Because I read one part of your post, got very distressed by it, and didn't see a later bit. This is my mistake and I'm sorry for that.
Why that happened: Some parents mistake a heavily-masking compliant child for a well child and unscrupulous ABA "practitioners" encourage that belief. The part of your post where you talked about making the hairstyle harder to remove came across to me as compliance-enforcing measures. I got worried and sad that you seemed to be doing that, so much so that I didn't fully parse the later bit where you empathised with DD and asked for "tips to help her manage". Again, my mistake. There was no reason for me to reply immediately whilst still distressed and I should have waited and re-read your OP.
Second reason: Because my job requires me to do a lot of fault-finding and not to assume that "the obvious" has been done because frequently it hasn't and that thinking has spread into my everyday interactions. That, I make no apology for.
Thank you for the additional information. I'm also autistic although wasn't diagnosed until adulthood. I didn't want to assume that your DD's experience would be like mine. I was also updo-resistant because who wants a thin elastic tie pulling some hairs more than others and pulling your whole scalp backwards and making the back of your head hurt? I was, and remain, generally styling-resistant across the board, because who wants a noisy hot hairdryer burning your ears whilst your mum pulls your hair with a brush? And who wants a stinky bright noisy salon full of strange people? Until I was old enough to comb and tie up my own hair, DM bobbed it for me. Once I could tie it myself, I could make it comfortable and had no more problems. My experiences informed the questions I asked.