Handling multicast traffic needing to flow across zerotier s2s

[Colt45]

I am using Cisco 2811 routers as essentially VoIP endpoints. Analog audio goes in, IP Packets come out. Thats all they do* The session target for the VoIP packets is a multicast address as it needs to go to all of the endpoints simultaneously.
Originally I was using all UniFi routers as the real internet-facing router. Their built-in IPSec S2S VPN was used to then link the sites.
To get around the multicast not passing over the VPN, I got GRE tunnels set up between the 2811s.
So there is a VPN between the sites, but then also a GRE tunnel within that between the Cisco routers

I have gone through, trashed all of the Unifi stuff, got it set up with OPNsense running on 1u servers, with ZeroTier as the S2S.
I would like to eliminate the GRE tunnels, as Ive found them to not be reliable. I have to reload the cisco boxen at least once a week to force the tunnel to come back up. Part of the reason I went with zerotier was because it was basically meshing and eliminating the single point of failure but that is maintined with the GRE tunnels.

What I would like is for the multicast to be transmitted from whichever end point, as multicast, then the OPNsense/Zerotier handles getting it sent out to all the recievers. What is the best way to do that?