It would seem that a consistent level of estrogen would be beneficial for both epilepsy and behavior. The slow drop in estrogen during perimenopause could affect both.
I have a 65 year old disabled sister (similar behavior to Down Syndrome) who WAS on oral contraceptives to prevent pregnancy (a sign of the times, now they use progestin).
My life experiences lead me to think that menopause, and perimenopause, would be difficult for an adult with autism. I’ve worked with children with autism and changes in their health can trigger unwanted behavior. Menopausal side effects would surely do the same. Depending on the type of seizure activity, high or low estrogen can cause problems. It seems that a consistent level of estrogen, perhaps low level, is most helpful.

Considering recent reviews of the safety of estrogen, if I were you, it is something I’d pursue with the doctor. Some doctors start estrogen during perimenopause. I’m not sure what the side effects would be of easing off progestin. Lots to consider. Bless you for all the years of care and love you’ve given your daughter!

There is an Autism group here on Mayo Clinic Connect. And I’m pretty sure there are adults with autism in the group. They may have menopause experience that would be helpful!

Best of luck as you work to figure this out!