Use nftables to repeat mDNS/Bonjour packets across two different interfaces. Works for Google Cast/Chromecast groups!

nftables_mdns.conf

table ip mangle {
    chain prerouting {
        type filter hook prerouting priority mangle; policy accept;
        ip daddr 224.0.0.251 iif eth3 ip saddr set 192.168.2.1 dup to 224.0.0.251 device eth2 notrack
        ip daddr 224.0.0.251 iif eth2 ip saddr set 192.168.3.1 dup to 224.0.0.251 device eth3 notrack
    }
}

table ip6 mangle {
    chain prerouting {
        type filter hook prerouting priority mangle; policy accept;
        ip6 daddr ff02::fb iif eth3 ip6 saddr set fd00:0:0:2::1 dup to ff02::fb device eth2 notrack
        ip6 daddr ff02::fb iif eth2 ip6 saddr set fd00:0:0:3::1 dup to ff02::fb device eth3 notrack
    }
}

Nevermind, it actually was working but I forgot to allow the regular traffic between VLANs. I didn't see anything in nftrace because the interface was not in promiscuous mode. Here's my working config:

table ip mdns {
    chain prerouting {
        type filter hook prerouting priority mangle; policy accept;

        # WARNING: nftrace does not work for this unless you put interface in promiscuous mode or
        # run tcpdump in the background
        # ip l set [iface] promisc [on/off]
        ip daddr 224.0.0.251 jump mdns
    }
    chain mdns {
        # repeat mDNS from IoT to main
        iif iot ip saddr set 10.0.0.1 dup to 224.0.0.251 device main
        iif main ip saddr set 10.0.4.1 dup to 224.0.0.251 device iot
    }
}