Smart playlists are broken after moving to a new Mac

I have a comprehensive music library with a not significant number of customised smart playlists. My 10yr+ old iMac started its death throes recently, so I moved to a new Mac Mini.

Initially, I copied my music data onto the Mac Mini, but when I opened Apple Music, I could see my library of music but although my smart playlists were visible in the sidebar, they were empty.

I deleted the music from my library and re-added it, this time choosing the Apple Music > Import option. Once it was completed I again could see my library of music -- but my smart playlists remained empty.

I deleted my music library again, and then tried the Apple Migration Assistant. My music data was again copied over, and my smart playlists could be seen in the sidebar... And yet they were still empty. (And my preferred profile was deleted as part of this, but I worked out how to get it back.)

Now I'm back at square one: my music is on my Mac Mini. Apple Music can see it. My smart playlists are visible, but are empty.

If I go to edit a smart playlist, I get this error message when I then click OK:

(I'm clicking cancel on that dialogue box, by the way, because I'm not sure what I'm doing.)

I sync my playlists with my iPhone, but haven't done so since I changed Mac because the smart playlists on my iPhone are currently working and I don't want that to break. I rely on the music on my iPhone to help me in crowded or uncomfortable situations, so I really, really need those to keep working. Therefore, I'm not syncing until the smart playlists in Apple Music on my Mac Mini are working correctly once more.

Does anyone have any suggestions, please? All I want is for my smart playlists to work again in Apple Music, and I've been trying to get this working for over a month. Thank you.