Since the same issue happens with other sources, this likely points to an issue with the router. Is the router on the latest firmware? I have seen some settings like Smart 2.4/5Ghz switching or EasyMesh cause similar behavior in the past, if you haven’t tried to do disable these it could be worth a try.
Hello. Connecting problematic Harman Kardon Citation One (MKII) speakers with a cable is impossible by design. Audio transmission is only possible over Wi-Fi. The speakers have a usb-c connector, but it is only for service.
I also report that I tried to transmit sound via Airplay not to 10 speakers at once, but only to 4. The sound stop was preserved.