

I don’t know how many mails you have. But if that is what you want, then do the work… Import it into Thunderbird, select all mails in a folder, give them a tag, and move them where you want them. It might take a little time, but my guess is, that it will take way shorter time, than trying to find different programs to help you do it your way.
Oh, and maybe you could come up with a different system, than that google put on you… Just a thought here.

I can only applaud it. And a nice cleanup once every decade feels good too. :-)