School staff may not have engaged with your concerns around possible neurodiversity because they do not see the side of your DD that you see. Schools tend to focus on whether they are coping with the work and if they behave well. It sounds as though this is the case at school, then the problems start out of school. She could be neurodiverse, but that’s a long road to investigate and you need something shorter term to manage things in the meantime.
Taking all of your posts together, it does seem that she struggles with larger gatherings and endings. Maybe after school when she’s already tired makes her less able to cope with her feelings.
It would be cruel to ban further parties if she loves them. So I would go to the next one expecting her to cry.
A few hours before the party
(not in the car on the way there) I would set expectations early:
”We are going to the party at 2 o’ clock and we will leave at 4 o’clock.”
Give her one reminder ten minutes before you leave the party (too many countdowns / reminders might make things worse).
Also give her a role that will distract her in those final moments:
“Your job right now is to find your coat and thank the birthday girl. Off you go…”
Then when that’s done, leave. Respond to tears with something like:
“I know. It’s hard when fun things end.”
So she feels heard. Don’t try to stop the tears. Don’t give her five more minutes. Be empathic but firm.
Then just drive. Let her cry it out.
When she’s calm later you can ask her what she found hardest about leaving.
Remember you’re a good Mum, and it’s quite normal to feel a bit worn down by this, but you will find a way.