Not sure if this is just something that's planned to be added later, but I really wish there was a way to reliably increase mental stamina when it gets low, before they become depressed. Physical stamina has the spa treatment option, but there seems to be nothing for mental stamina except cancelling everything they're involved in (if you catch it in time, which is usually tough to do unless you don't have any of the screens open) or just putting them on hiatus and accepting the losses. It'd be nice if there was a similar option to the spa treatment (maybe a trip to the counselor? or just a day off? lol) or if the doctors could have an additional function (therapy/talk sessions for the psychology-focused ones and something like health classes/physical therapy for the physical ones).
On the same line of thought, are the variety show proposals supposed to take mental stamina instead of physical? It doesn't say so in-game that I can tell, and that was my downfall a couple of times until I figured it out and just stopped accepting variety show jobs. Now that I maxed out the producer's variety show stat, it usually takes over 100 stamina, which is enough to instantly depress an idol. Very frustrating.