I tried that recently. I didn’t like that it doesn’t have a widget, and the downloads and current playlist are completely separate. Also there was no option to automatically continue when connecting to a headset (this was working in Ultrasonic 4.8, but not in 4.7.1 I hope they fix that bug sometime…) So after some weeks use I switched back to Ultrasonic.

My offline android music workflow:

• Server: Navidrome but any music server supporting Subsonic API would work here. Navidrome has a nice UI, and reads MusicBrainz IDs, and can scrobble to ListenBrainz, that’s why I settled with this.
• Mobile app: Ultrasonic, on Fdroid. There are a lot of ways you can set up caching. I set up that it should automatically download everything from my “Now playing” playlist, at home on wifi I just add a bunch of albums and playlists to the “Now playing” list, it takes a while but it transcodes and downloads everything in a couple of minutes. It has very good Android Auto support, and a widget. Due to an annoying bug I had to downgrade to version 4.7.1, but otherwise I love it.