I like
Cynthia a lot and its definitely underused. I like Forsythia even more though. I don't consider it dated, because it did not become popular enough to have a strong decade association. That said, the
Cynthia I know is in her mid-fifties. She doesn't go by
Cindy. I think Cynthie is a cute nickname. Cit (like
Kit), Thia, and
Tia could also work.