Is there a specific reason you trust your router for IPv4 but not for IPv6 traffic? IPv6 privacy extensions should be enabled by default on most devices. So even in the unlikely case of a device being exposed, someone has to know the temporary IPv6 address and then try to access it while it is still in use. This device would also have to run a vulnerable service on some port that the attacker has to know. All in all, I think that this is a pretty unlikely scenario.