I haven't changed my router yet but I moved all my Bose Soundtouch speakers to the alternant 5G network on my WiFi router. They are the only items on this network.
I have monitored them for a few weeks now and we have not had one dropped speaker and they are always available in the app. So it looks like the multicast issue on my router was the cause as you suggested and other equipment was interferring.
So solution would appear to be keep them on a separate network where possible or select known stable multicast routers.