I'm glad we're learning more about Yuki and Ban's days as Re:Vale, I don't remember too clearly but I think only the accident was what was talked about in the previous two seasons. I don't remember these details about the constant changes of band members and then them turning into an idol group but if could have been mentioned in passing.
It makes even more sense on what Banri mentioned when talking to Tsumugi about Yuki's harsh words, he looked really intense in the past lol, he has really mellowed down although he still has a sharp tongue.
I'm just hoping with Momo revealing this to Ryou, Ryou doesn't try to set something up after learning Our Incomplete Selves is the song where the accident happens if under Momo's request, Yuki decides to sing the song with him. I do not wish for him to go through as something as traumatizing as that twice.