using an old android phone as a music player

Since your music is already there, you can also install Termux, inside it install and configure Navidrome, and use it as a music server to stream into your computer from it.
It would also be useful for adding music since you could also install and set up an SSH server on it, and rsync files easily.

This seems underwhelming.

What I find to be more compelling is to use an old android as a Sendspin client. Pick an open source music server of choice like Music Assistant and now you have multiroom sync players akin to overpriced Sonos devices, you just supply the amp/speakers.

If you really like fancy audio, just connect the old phone via the USB to a nice DAC like a Fosi K11 and you have a very reasonably priced multipoint streaming solution.