Multiroom with UPNP Renderer

[kosmonaut_75]

I am using Minimserver on a Synology NAS, have three Raspberry Pis with DAC hats connected to separate amps as UPNP renderer (with Moode Audio and UPNP plugin) and BubbleUPNP as control point. Works like a charm.

However, it is important for me to implement synchronized multiroom, that is the same music at the same time in three rooms. If possible, with an open source system on my Raspberrys.

1) I could do this with squeezebox, however, I definitely want to stick to minimserver because of the great tag browsing for classical music.
2) I could do this with Google Chromecast Audio, however, I am not happy with the audio quality, it doesn't support real gapless, and Google announced end-of-life of the CCA.

My question: Is there an open source UPNP-based solution for synchronised multiroom? I read something about OpenHome, OpenMedia a.s.o. but the documentation is almost non-existent and the downloads partly outdated.

Any ideas or help would be much appreciated, I am really stuck!