I would think that the free U2 album would not prohibit the migrate, since that would make it impossible for anyone, right? I've never even opened the Music app on any device while signed into the primary account for media/purchases, so I don't know if I even have it in there. So hopefully it's the TestFlight for you.
Yeah from what I've read, I think the data points show it is not the U2 album per se, but rather having ever either A) subscribed to Apple Music or B) subscribed to iTunes Match. If you just PURCHASED music (or received the free U2 album), then you are still OK. In my case, TestFlight removal still didn't work because of the aforementioned issue.