I went into the offline mobile music player rabbit hole recently too. I tried like a dozen and personally I think none of them are sufficient, including Symphony.
The most important functionality for me is queuing e.g. how it works and how easy it is to use and it’s just a horrible experience for virtually every player. So disappointing 😣
I’ve been happy with Phonograph for queueing, nothing special but it seems very easy to use to me. Looks like tons of good recommendations on this thread I haven’t tried out tho, definitely might be something better out there