Lottsa good thoughts here, and I don't have much to add, but it sounds like you know what you want to do, the rest of the group agrees (it sound like) and you want the parting to be as healthy for the child as possible.

Change her role. Have her do the invited "guest spots" that highlight her strengths and only bring her on at appropriate shows. Now she's a part time member. This gives you and her a buffer. She could be more easliy 'phased' out, Mom's happy, and you could 'phase-in' her replacement also by doing 'guest spots' with a new member (if indeed you replace her).

Sounds like this little one needs a push outta the nest. Don;t be surprised if she blossoms and flies away on her own, once she's out ... but I suspect from your concern, that would only please you, anyway.