Found a Japanese blog rating Kohaku chances. For the blog, 7.0 is the line for "possible" and 8.0 for "likely". On the anime slot Suisei ranks 6.3, so doesn't merit a closer look in the blog, but I guess his evaluation criteria give some insight what NHK might consider before sending out invitations.
On the other hand, for the regular musician side, Suisei ranks 6.9 which is just below the possible line:
https://aqours-with-anison.info/archives/27061612.htmlSeems like it'll be a month for the official announcement.