

That is an excellent episode. It would probably have been my fourth pick. That or Arena, the Gorn don’t get enough love and while the episode is cheesy (more so then the rest of TOS), it’s a classic. But I had to go with Mirror, Mirror. The mirror universe episodes in any trek are some of my favorites.
Also, for at least two of those three the cause of their graduations seem to be chronic medical issues. IIRC Sana had some chronic back problems that at least for a little bit rendered unable to walk and while it got better after that, it didn’t go away. I think she also had several personal tragedies happen in rapid succession.
Mumei has said she has some chronic throat problems that have meant she has a hard time talking for extended periods of time and a harder time singing.
They’re also not the first gen in Holopro to lose half their members the first generation of Star EN lost two of their four pretty early on.