AFAIK IPv6 multicast across the internet is pretty much dead. ISPs seem to block it because of its DDoS potential. They use it themselves of course (very useful for streaming live TV across their private VLANs) but as an outsider you'll have to convince every ISP and backbone provider to trust your multicast stream, which they probably won't.

Tailscale (or any other P2P overlay network) could solve this problem by re-enabling the multicast support that most ISPs block. It's not a terrible idea.

Edit: a comment elsewhere linked https://www.librecast.net/librecast.html which seems to be doing exactly this.

Transit and peering agreements between ISPs typically exclude multicast, meaning packets are dropped at network boundaries.