"why did you make a solo debut?"
"when I left my old agency for the new one, I was the only one there." - video below
TLDR: There is evidence that Somi might have left JYP because she didn't want to join a JYP group, not because she wanted to go solo. She joined TBL because Teddy is great with solo endorsements (like with BP). She went solo because TBL trainees were not ready and she needed to stay in the public eye, but when they are ready she might join the group.
This will likely be Somi's last chance to ever be in a proper (not project) girl group, but if she joins the group she can still do solo work in the group and after disbandment (because it's a Teddy group). She must be considering it.
Despite this, I don’t see anyone even considering the possibility that Somi might join Teddy’s new group (from The Black Label, not YGNGG ‘Beamon’). Perhaps because Somi is ‘too famous’ or she is ‘only interested in her solo career’. Maybe, but I don’t know if either argument is as strong as people think.
First, Somi appears to be open to being in a group. She has stated repeatedly that she misses being in a group and is sad that all the groups she was in lasted such short periods of time (four project groups so far, although the last one is just an ad). When asked why she didn’t join a group after leaving JYP Somi said it was because TBL didn’t have anyone to debut with (but it appears that they do now...), not that she was only interested in going solo (video above). People assume she left JYP to go solo but there are plenty of other possible reasons, including losing all solo endorsement/TV opportunities (she had many) if she joined a JYP group.
Second, the ‘too famous’ argument is entirely based on Miss A as far as I can tell. The situation would be completely different if Somi was to join a group (also there's Ive and Lesserafim). Somi is already famous before joining the group, so there is no reason for the other members to be jealous. Somi’s contract wouldn’t share her solo deal money with the group, avoiding the tension that creates (same with the other members' contracts). Somi would also be much better than Suzy at supporting the other members through their relationships, like Somi and IOI members, or Somi and Chaeyoung, or Somi and Giselle… etc.
Somi has been teasing 'crazy news' since last December, around the same time she showed some of TBL's trainees on her vlog. There's a small chance that the crazy news is that she is going to join TBLGG. Whatever it is, it's been in development for a long time.