I admittedly didn’t look super closely at the projects that have been linked, but my guess would be that the phone can be replaced with SW on the Pi. Something like volumio that was linked, or Navidrome for music.

Mapping was always kid of a mess when I looked at this kind of thing in the past, and I don’t think it has gotten any better.

If it’s the phone hardware you don’t like, rather than the software, there’s always LineageOS on a Pi (konstakang.com/devices/rpi5/LineageOS22/) as an option. Then you can still use whatever SW the phone would have had on it.