Moreover, if I understand correctly, PCP is the only option for IPv6. So I'm waiting for PCP support.

Why not to mark packet at mangle postrouting? It does only make sense to use FastTrack on specific (known) connections before they enter filter/other routing chains. Using it afterwards makes no sense at all... We mark whole connection. It is better mark first packet (when connection state is new),...

Why not to mark packet at mangle postrouting? Like this: All traffic already have filtered and we also mark new connection.

Switch chip AR8327 have builtin hardware igmp snooping support. How to enable this feature?