The mdns-bridge package seems to be a feature-complete and up-to-date implementation of mDNS forwarding across interfaces for both IPv4 and IPv6. It handles only mDNS as opposed to the general-purpose (and IPv4-only) UDPBroadcastRelay, but at least it should make it easy for mDNS. It's getting harder to live without IPv6 nowadays, due to IoT gadgets, Apple and Google insisting on it.
There's no OPNsense plug-in for it, but it is in FreeBSD ports, so it should be ok to try. Anyone have any experience with it on OPNsense 25.7 or 26? Just looking for any concerns before I end up flooding everything :-)