I am, as always, approaching your non-autistic DD with my autism toolkit, but here are the tools I see as useful.

1. Ask the school for a Functional Behavior Analysis done by a Board Certified Behavior Analyst. They should agree to this; they do not want disruptions to continue any more than you do. The BCBA is the person best equipped to take data and notice all the trigger situations that create anxious/oppositional behavior, and to set up a plan to systematically change your DD's reactions in those situations. They work precisely on that moment between anxiety and opposition that you have identified.

2. Consider private ABA (Applied Behavior Analysis) therapy, again with a BCBA. These folks do serious behavior modification in a way that works. This should be coordinated with your CBT program. The CBT works on the thoughts that aren't useful, the ABA works on the behaviors that aren't useful. Some practitioners do a blend of ABA and CBT, but this is rare IME. Neither is a fast fix, but I would bet that ABA is faster for a 5-year-old.

3. You need more than a 504, you need an IEP. I think this because your DD will need extra services to work on generalizing her new coping skills and newly learned behavior strategies throughout her day. The FBA would tell you at what times of day she needs this most; a special educator deployed specifically to prompt and reward the desired behaviors is the main way this job will get done, and you could then fade that support as soon as the skills were mastered. As long as she finds her coping strategies more rewarding to her than doing what's desired by the adults around her, it will be very hard to get anywhere.

4. I would talk with a developmental behavioral pediatrician about anxiety meds. For some kids (including one of mine) relief from anxiety improves everything and radically reduces the (seeming) oppositionality. It can truly be night and day. For my DS, anxiety meds gave him the ability to access the behavioral therapies we have used; without the meds he was in fight-or-flight too much of the time for any of it to be useful.

To be frank, I don't see all this as "slightly 2E." Maybe from the testing perspective she only measures as slightly 2E, but it sounds from what you write as though she is always on a knife's edge of being too distressed, and it sounds as though it affects her throughout her day, nearly every day. I think that after you start addressing the anxiety, you will be able to get her to the point where she realizes that the more advanced schoolwork is fun; but I'd work on the anxiety first.

DeeDee