Maybe I fooled you by using the term 'router'.
It is an usual OpenBSD box installed on a normal amd64 arch. that does NAT for my SOHO network. It's not a router device itself.
Its a router because it routes traffic. It doesn't really matter what the hardware and software are. If it routes traffic, its a router.
As has been said before, you don't need to fiddle with NAT for ipv6. With your tunnel you were given a "Routed /64". This is the subnet/prefix you will want to use on your internal LAN. Just configure your OpenBSD machine to run radvd configured with the "Routed /64" prefix on the internal interface, and ensure that the machine will forward ipv6 properly. Then all your internal systems should autoconfigure themselves with an address within that prefix and be able to talk ipv6 to the world.