Google released a handy tool to transfer your Google Play Music stuff to YouTube music:
It “may take a few hours”, but from the look of things that’s still quicker than what it takes me to find a proper alternative to YouTube Music. For what it’s worth, I did install Ampache server at home after trying some alternatives… but then life happened, and I haven’t had time to actually config it yet.
What I did do is rediscover the tersely named Google Play Music Desktop Player (GPMDP for short), which is essentially an Electron wrapper for the service itself, and now has YouTube Music (YTM for short) support too. Apps exist for Mac, Windows, Linux, and the Linux app for one integrates nicely with the media controls of my Linux Mint.
What it doesn’t do (because that’s a YTM thing) is disable the annoying “automatic next song” feature, so I end up auto listening to rando Dutch household EDM after a GoGo Penguin album, which is exactly why the search has to continue for a proper alternative.