Aren't the only kpop soloists that have been truly successful in the West Psy, BTS and Blackpink members (and Psy's case was meme fame)?
So, if Somi achieved true solo fame in the West it would be a first.
Other big names in kpop have tried. IU seems to get it and hasn't even tried. Perhaps it's understandable that Somi hasn't achieved it yet.
Currently, Somi does seem to be the most internationally popular female kpop soloist except for IU, Nayeon, and BP members, but Somi is about 4-8 years younger than them so she has plenty of time.
As for Loona, an 11-12 member group is very unlikely to ever go mainstream in the West, irrespective of management.