Snapcast is available from the LibreELEC Add-ons repository. Install the Snapserver addon on one system and the Snapclient addon on all systems which should play. Then use AirPlay to stream to Snapcast.
By default, the Snapserver addon is configured to stream AirPlay, but it can be configured to stream Spotify. Streaming Spotify requires that the Librespot addon is installed on the system which runs the Snapserver addon.