I need to announce the Holy Grail of multi-room music streaming is at hand.

Hardware: ESP32+DAC modules, available with line-out or speaker amplifiers, starting below $20 each https://github.com/sonocotta/esp32-audio-dock

ESPHome YAML calling in a PR that implements the Sendspin protocol: https://github.com/esphome/esphome/pull/12284

Music Assistant either standalone or as Home Assistant addon: https://github.com/music-assistant

This stack gets you access to most music providers, and perfect multi room sync on wifi.