First of all I’m so sorry you are dealing with this! I don’t think it’s unreasonable for you to cancel her party and hold up the boundaries you’ve set - especially since it sounds like you cannot trust her reactions and don’t want her to be set off in front of her friends. If she acted the way you are describing in such a public situation that may have longer term consequences for her friendships.

I think it’s reasonable and you should to still celebrate her in a more intimate, controlled setting. Maybe she still gets the one-on-one special time with you, but not the party. That way she knows she is loved & cared for, but neither you or her has to deal with the stress of the larger social interaction

Coming from someone who has dealt with childhood anxiety, I can vividly remember times when there was an event I needed to be present for, but would’ve rather just been home alone without the pressure of “behaving normally” for my friends. I usually enjoyed social situations, but during the times when I was struggling, they made things worse.

If you do end up cancelling, I would just give the other parents, a vague, she isn’t feeling well excuse as to not embarrass her