I have what may only be a similar problem. We bought a set of HomePod minis in our effort to get rid of Alexa. My wife bought them under her username to take advantage of the 6-month free music subscription. When I realized it's cheaper in the long run to use the Apple Music family plan that (supposedly) would allow us to share music and, I expected, use a common library I had her quit that subscription after a few days and signed us up for the shared plan instead (4 bucks a month cheaper too).
It should have been no surprise to find that actually sharing anything except the license itself can be a nightmare. In typical Apple fashion I had to cruise these help forums and search for answers since Apple is never forthcoming with setup instructions, requirements, and troubleshooting advice. Lots of fanboys have disagreed with this but those that do have never drunk another brand of koolaid. But I digress.
What I found was a slightly older article that says we can only share music that was purchased or leased from Apple. Seems to me the only reason for this is to make it more convenient to either lock your new account into giving control of each family members content over to Apple in typical Apple fashion. Since my library is local to my system and hers there is no technical reason why this should be necessary. If this is true then I will have to go through everything I have that she maybe interested in and re-upload it to her (supposedly) shared account. Thereby increasing the load on our also shared iCloud account by doubling the space necessary.
I haven't yet tried the techniques offered here to give her access to the content I am supposedly able to share but, if true, I have a lot of work to do to make several DAYS worth of music I owned before I came over to the dark side. And, yes, the longer I use it the more I come to understand that it truly can be dark over here.
So, to make a short story long, I invite someone here to tell me how wrong I am about being able to share all my music content with her...I look forward to the help that Apple should have supplied in a file full of knowledge along with the subscription.