There's a fan documentary that pretty much tells you everything you need to know about Suisei, though the relevant bit of info is that she was a unique case of a Vtuber getting admitted into an agency without relinquishing her identity as Hoshimachi Suisei, and along with that all of the business connections she made as an indie.
It does give a bit of a hint as to why someone would want to join an agency rather than continue being an indie.
As for her single
comet, it's in the video description - she provided the vocals and animated the MV herself.