Navidrome looks nice but it looks like it is Desktop only. I am using Plexamp as well. I tried some alternatives but couldn't get them to work reliably. People miss Plexamp as an option because they try the regular Plex app and not the simplified Plexamp.
Navidrome is a server. You can use it directly by connecting to it in a browser, but its point is to serve the music to a client, of which many exist for both iOS and Android.
Navidrome is a server, not a desktop app. It's more analogous to Plex than Plexamp. If you want a mobile app that can do Subsonic (the protocol Navidrome uses), Symfonium on Android is amazing.
Navidrome is compatible with browser apps through a protocol the name of which eludes me at the moment
The web interface also runs pretty flawlessly on mobile browser. I was actually shocked at how responsive it was.
Subsonic API: https://www.subsonic.org/pages/api.jsp