I'm a school psychologist and mom of a 2E - my advice is to keep her on a monitor IEP.
Reasons:
-she crashed before after a good year and she does have a disability that'll crash;
-she hasn't even made it to high school but is close; her gifted classes should become more challenging, one hopes;
-re-entering school at the beginning of the year and exiting sped at the end of the year - that is too short a time frame to guarantee it's going to be in her best interest;
-since 504 is unfunded and IEPs are funded, you tend to get access to help if needed more quickly than with 504.

I hate to think about this talk about attorney's fees, and I say that as one who has paid 'em and gotten help thru OCR. Not my favorite way to decide between IEP and 504.

I'm more concerned about what's going to help this child. By the way, "good" girls with internalizing disorders get almost no mental health services in public schools, while as adults they are the largest category of patients. Could your daughter benefit from help with language pragmatics, self advocacy or stress management? Anxiety disorders do benefit from psychotherapy, which is something I have provided quite often as a school psychologist. There's a huge nexus between mental health issues and sensory issues with ASD, so a good occupational therapist can be part of the IEP services.

Now I don't know your daughter, but I've always said emotional disabilities are chronic and fluctuating. You have to be able to stay ready for the next high and the next low. Not get to the high and exit the child!