I paid for Plex, but then they broke the the downloading features from the server to the Android client, and never repaired it to work reliably.
Meanwhile most of their updates were about streaming support, and then they started cramming their streaming service into it, and pushing it, and I just got sick of all of that. Eventually I just switched to jellyfin. It is far from perfect. The music player isn't as good as plex's, there is no download feature. But at least it hasn't turned on me yet.
Jellyfin's Android app does let you download files but having to do music tracks one by one isn't very useful.
Finamp is the app to use for proper offline playback/sync of music from your Jellyfin server. Go for the beta version, it's far ahead of stable and works well.
It's still poor after the new design. If I downloaded N episodes for M shows, it shows me N*M episodes all at once. No way to say "Hey, filter this list to just episodes of show M, Season K".
Meanwhile most of their updates were about streaming support, and then they started cramming their streaming service into it, and pushing it, and I just got sick of all of that. Eventually I just switched to jellyfin. It is far from perfect. The music player isn't as good as plex's, there is no download feature. But at least it hasn't turned on me yet.