All the big music streaming services now have family plans that allow you to share your subscription with others to save yourself a little dough, but they each vary a bit in how they work. Conclusion: If You’ve purchased iOS devices for family members that are being used on Your ‘family sharing plan’, remove the ‘association’ in iTunes because Apple Music has a problem with that whereas family sharing for apps, music and books didn’t seem to mind.
I am also in Product Management, and understand these issues, but someone has made the cumulative decisions that have led from a small family domain that Google encouraged me to use to get the latest features being treated as a corporate account that doesn’t allow the latest features.
It simply should not matter whether; family members happen to own their own business (I do), that some people have grandfathered Apps accounts (I do not, but many people here do), or that some or all of the family are using an apps account (many do).
Spotify works on our Rokus too (Google does have a Google Play Movie/TV app on Roku, but not a Music app), same goes with my Fire TV. We also have several Google Cast (formerly Chromecast), Spotify works with that too (as of course does Google Music).
I didn’t read all of the previous posts so if this has been suggested I apologize… What I have done for my wife and me is created another user account and I pay for play music on this account and we add that account to our devices and access music through that account.