Oto Music (Play Store link) has been my mp3/offline music player of choice for years now. Stable, pretty, performant, and has tag editing features built in. Still gets updates at least once or twice a year, which is all ya really need for an offline player.
Megalodon (Play Store link) is my Mastodon client of choice, tons of little ease-of-use improvements over the official client. Some people might already be familiar with this if they’re on Lemmy, but maybe someone looking for a better Masto client will get something from this.
Ah yeah it would be nice if it were open source. Retro is pretty good, I remember having stability issues with it when I used it a couple years ago, but if those got ironed out that’s awesome.