LMS would fit the bill perfectly. LMS, or Lyrion Music Server, formerly known as Logitech Media Server, is open source software that's been given to the community by Logitech themselves. It used to be used to stream music from your computer to Squeezebox devices, but now you can stream it to nearly any computer, including...

Raspberry pis! By using piCorePlayer, you can turn your pi into a server for LMS, a player, or both at the same time!

I've been using LMS for years and can't praise it enough. I've got whole house audio. My server runs on a Pi 4 and streams music from my NAS, but you can even stream it from a connected drive, Spotify, Deezer, or Qobuz!

Also, don't fret about LMS's...dated...UI. There's robust themes out there, including the legendary Material Skin https://github.com/CDrummond/lms-material

Since you can use a Pi, you can connect any amp and speakers to it. Hifiberry makes great DACs that you could use for a cleaner sound, but any modern USB DAC would do well IMO.

You can still get Squeezeboxes cheaply on eBay, BTW. The Squeezebox Boom and Radio both are all-in-one WiFi enabled devices that can connect to your LMS server and stream music. Their sound quality is excellent, even for 10+ year old hardware.

If you have any questions, hit me up or check out the forums: https://forums.slimdevices.com/

https://lyrion.org/ https://www.picoreplayer.org/