Yup, I agree with sbm. Also I have to say, you have a very opportune timing with your question, because I found another tidbit of information that can help with the timeline. In Pomu's last totsumachi while talking with Maria she told that Maria was one of the first to find out she was graduating, the reason for that seems to have been mentioned in another one of Maria's streams where she told that she has been working on a 5 person cover that includes Pomu, and that when she approached her back in July to talk about it Pomu mentioned that it would be releasing after she had graduated. So it seems that Pomu made her choice about graduation in July or earlier, which seems to fit with the timeline.
I have to say though, this whole timeline thing has made me think and reflect about the long time that passed between Pomu taking the decision and the actual graduation. I don't know if she chose this date or if it was set by management, but it hurts my heart to think that Pomu had made her choice so long ago and still she carried on streaming, even though the choice to leave was already taken. Makes it hard to go back on Vods from the last half of the year and think that while she seemed happy on the outside she was actually preparing to leave.