So, I really hope this doesn't turn into a Descendants reunion (because I also have a Descendants actor in mind for one role, and Dove Cameron is my go - to for at least two of these), but for May Singer, I chose Anna Cathcart.
I've seen her as both Olympia on Odd Squad and Dizzy in Descendants 2 and 3, and I really think she would capture May's enthusiasm and spunk.