So it's the Apple Music subscription that ruins it?
The support article implies that is possible if the primary account already had a music library...
Learn about what purchases and associated content can be migrated between two accounts.
support.apple.com
Though that conflicts with the other article
If an Apple Account is only used for making purchases, those purchases can be migrated to a primary Apple Account to consolidate them.
support.apple.com
Turns out my primary account had that stupid free U2 album, and nothing else.....