Volumio
Will run on a dedicated raspberry pi and x86.
(dedicated means, the computer will only do this, nothing else)
Connect the sound out of the device to the amp and it will provide a web interface to directly control playback.
Music is not streamed, it can play files from a local disk, or from a network mount.
The free tier is 99% of what someone needs.
Moodeaudio
Same as above without fees, though explicitly only for raspberrypi and a limited number of SBC's
A key fundamental for any music is the metadata tagging and Musicbrains Picard is the best way to do this
Picard can automatically find the proper metadata for almost everything, and, can "scan" ripped from cd tracks and automate the naming and folder structure that a music library should be in.