You sound like a very caring parent. Your daughter is lucky to have you helping her!
There is a discussion on Mayo connect for adults with autism/ASD. It is not specific to cancer but they might be helpful in finding some solutions for you from their personal experience dealing with medical issues.
https://connect.mayoclinic.org/discussion/seniors-on-the-spectrum/
It seems there are multiple levels to your situation. There are the cancer treatment side effects that members might help provide suggestions on how to deal with them, there is the issue of autism and possibly communication or anxiety or difficulty adjusting to new requirements including doctors/treatments/places, and then also being responsible for her care. It’s a lot!

This article from the American Society of Clinical Oncology,
https://dailynews.ascopubs.org/do/autism-and-cancer-creating-comprehensive-solutions-complex-needs, has some general suggestions that might be helpful. Although, after 23 years of caring for your daughter you may have already tried what they are suggesting. I don’t want to be yet one more person providing the same answer you’ve heard so many times before.
They do recommend creating an Individualized Oncology Support Plan (sounds like an IEP, right?) But maybe there is something on their list that would be helpful for your daughter.
An excerpt:
“One means of accommodation available within an IOSP is the provision of coping support through cognitive picture rehearsal, pioneered by Groden et al. Cognitive picture rehearsal is an instructional strategy that uses functional behavior assessment to identify antecedents or stressors particular to the person. A positive coping scene is created according to this information and is designed to train a more adaptive manner or response. In addition, stress-reduction techniques, such as relaxation training, that combine behavioral and cognitive elements are highly effective. These tools help change attributions and provide learning mechanisms for self-control that the person with ASD can access as needed.”

I’m sorry I can’t be of more help. I have an adult sister with a developmental disability, not autism and no cancer, so I have a small inkling of the challenge you are facing. I hope you can take the time for the self-care you need. I’m sure you are on call 24/7, 365 days a year.