Ooh ooh, I know this one!
You could run Mopidy, which has support for Subsonic libraries. You could also run plain MPD.
Whatever you decide to go with can then be connected to Snapcast, which is a server/client setup for streaming audio from a source to multiple client endpoints (in this case your workshop, phone, PC, etc).
On devices that can run the client software, like a desktop or phone, you just run the Snapcast client software.
To connect stereo/AVR systems to Snapcast, you can build a streaming endpoint with a Raspberry Pi ZeroW with a Pirate Audio hat, or the version without the screen, and set up the Snapcast client software on it, and then connect it to your stereo system.
If you have a 3D printer, you could optionally print out a case for the client devices.
This is my setup, right down to using Navidrome as the Subsonic server and I couldn’t love it more!
No problem. This is essentially what Sonos charges hundreds of dollars to do, but ends up costing ~$150 for the server and ~$35 per client device (using Pirate Audio + RPi ZeroW). One thing I neglected to mention is that if you happen to have Spotify Premium, you can set it up so that Snapcast becomes a Spotify Connect output