I think I found a solution, or at least a workaround. Here's the TL:DR version: Create a new music library for the desktop app.
I created a new music library (directions here), made sure the new library was in the "music" folder in my user folder on the MacHD, then clicked OK when prompted to merge the library, which brought back the playlists and files from my previous library, which was on an external drive and the one I had merged when I first started the streaming service.
The process
This was not the solution Apple support hit on, though they were perfectly pleasant. They had me doing the usual troubleshooting moves (that I had already done on my own but whatevs) like restart, reboot in safe mode. Then the support tech asked me to upgrade to Sonoma, which is where I stopped the call. Upgrading to an OS that's been out for only a month—and isn't yet tested with a lot of my music studio software like Universal Audio and others—is kind of a nuclear option.
But because the support tech tried looking for my music library in my user's music folder (even though I said it was on an external drive but she might not have caught that), I decided to try creating a new library in that folder to see what would happen, and the app launched. I was immediately prompted to merge the library, which brought back all my old playlists and files and allowed me to switch from streaming playlists to previously made playlists without crashing. All good! Almost…
The aftermath
As a test I tried to add a new track from my computer to a playlist of rough mixes, which threw up a prompt that said I had to remove that playlist from the Cloud Music Library. OK fine, whatever. I don't need my personal files in the cloud, I just need them in the Apple Music app to send them to my phone so I can check mixes on various speakers and in the car.
Also: I went back and changed my media folder in the desktop app back to the external drive, because that's where I want the new files I add to the app to be stored. I don't need all of my rough mixes and client music clogging up my system hard drive when I have terabytes of storage available externally.
More aftermath: logging in and out of my Apple ID when I was troubleshooting really screwed up a bunch of other settings on my phone and computer, so if you did that in your troubleshooting journey check your wallet, photos, and g-d knows what else. Because all of a sudden the credit card attached to apple pay was disconnected, meaning I couldn't pay for a subway fare until I figured that out; my photos app somehow switched back to cloud storage which I never use and don't want to use and prevented me from deleting old photos in Image Capture. So much time wasted. This free trial has stolen 8-10 hours of my life and so much unnecessary aggravation.
So, if any of you out there have your Apple Music library on an external drive and Apple Music streaming crashes your desktop app, then doing this whole song and dance with a new library might be a solution, or at least a workaround to get you going again. Oy.
