I'm not sure what the problem is, the standard home router provided by my ISP does the same thing for both IPv4 and IPv6, only without the NAT, but with exactly the same firewall rules. Inbound connections are blocked unless the port is opened by UPnP, or manually opened in the router's GUI. I assume this is how most home routers work, or am I wrong here?

I don't really see why UPnP would be bad either, it has become a well-known and industry-standard protocol, and is the only protocol that has managed to enable a reasonable, configuration-free firewall for home users. Even though it is currently commonly used together with NAT, all fundamental problems in the NAT/UPnP setup are due to NAT and not UPnP.