Currently I'm using Jellyfin (since I already use it for other stuff), but it isn't ideal. No option to download entire albums easily, and if the server is offline then I can't even use the music downloaded to my phone.
I use Plexamp with a lifetime Plex Pass subscription and love it. I mostly use it steam and keep some sonic analysis based radio stations downloaded for offline use.
I use Plex with Plexamp and love it other than the forced online account, which is minor enough in my opinion that it's been hard to justify looking for an alternative. What did you move to?
I moved to Emby, then to Jellyfin. Also, the other thing that bothered me was that with Plex I noticed I had an online account where I could access all my files. In other words, Plex was using my local data, which was the straw.
Yes there's something particularly sketchy about an app that makes those kind of anti-user corporate decisions when it owes its popularity to the piracy community