Hey, totally understandable. I've almost done things like that too.
Perhaps if you combine Twilight Project with Doki Doki, you can limit people to two roles each. For example, some could be both Airi in C-ute and Koharu in Kirarin Rev. Maybe larger roles (like Takahashi Ai in Morning Musume) can count twice (or you can double-cast the roles) and very small roles (such as Akari in Eggs) may not count fpr a slot at all. This will open up some roles for new auditionees.
Then pick a leader for each group, preferably someone other than the project leaders (though they can run a group if they'd like) so that you spread out all the leadership work.
Let me know if you need help with this.