Has any Trek really had a “mainstream breakthrough” since TNG?
Not really. Indeed, I’d say TNG wasn’t much of a mainstream hit outside North America. Kelvin films did it to a degree. I think Disco did have a bit of early success outside the US; certainly in Australia it was bigger than I expected it to be.
To put it another way, there’s nothing to suggest that S3 of Picard has attracted substantially more viewers/subscribers than any of the previous seasons of Picard or Disco, such that a spin-off is obvious.