This is exciting. About 5 years ago I tried to set up a home-made Sonos clone by using two Raspberry Pis to stream synchronised audio across my network. I did get it to work, but it was a huge hassle finding the particular combination of PulseAudio configuration flags to use, and I had to set up a dedicated wireless network for the bandwidth because it fell over if I used compression. I figured the best approach would be to write a PulseAudio module in C but I never had the time or skill to do it.
I'll definitely be giving this a go!
I've built something similar at home with three RPis and Snapcast [0]. It has an integration with Librespot [1] that shows it as a Spotify destination. It works really well!
[0] https://github.com/badaix/snapcast [1] https://github.com/librespot-org/librespot