Another vote for navidrome. Easily self-hostable, and since it uses the Subsonic APIs, there's plenty of client apps to choose from (I personally like Symfonium[0] for Android).
If you're already using them, both Plex and Jellyfin also serve as music streaming servers (Symfonium will also work with these)
I use Ultrasonic as well, but I've found Symfonium to be much more stable, especially wrt Android Auto and Chromecasts.
There's several other open source Android apps as well. I've been testing out as many apps as I can find and documenting the features of each in this spreadsheet[0]. It's a bit out of date, and there's one or two more apps to add, but you get the gist
If you're already using them, both Plex and Jellyfin also serve as music streaming servers (Symfonium will also work with these)
[0]: https://symfonium.app/