The StreamPlayer Gen III and Gen III SE with the just released version of Roon now adds Chromecast to its list of supported devices. This will allow you to set up a series of inexpensive Chromecast devices and group them for playback.
The following is from Roon's announcement:
Roon now streams to any Chromecast device! This includes the Google Home family of smart speakers, Chromecast and Chromecast Audio receivers, and a wide range of third party products. In addition to Roon Ready, AirPlay, Sonos, Linn, Devialet AIR, Squeezebox, and other network audio products, this new features boosts the number of Roon-compatible devices into the thousands.
If you have Chromecast devices, they will appear automatically in Roon’s Settings screen under the Audio tab.
With audio-only devices (like Google Home, Google Home Mini, Google Home Max, and Chromecast Audio) you can now enjoy gapless high-resolution music (up to 96kHz/24-bit) from Roon. On Chromecast devices with video (like Chromecast Gen 1, Gen 2, and Ultra), Roon displays album artwork, artist photos, and track information in addition to audio.