You are not bypassing the router, the devices need to get their packets from somewhere, and it is only like a forever-open port if the router/firewall decides it is.

My ISP router supports IPv6 but blocks all incoming connections by default, which is kind of like what NAT does as a side effect.

It sounds like insanity because we tend to assume that no NAT means no firewall, because NAT has some firewall-like properties, and on the most basic networks, that's the only "firewall" there is. But none of the security features of "NAT as a firewall" are exclusive to IPv4, in fact, IPv6 has an advantage because the much larger address space makes a full scan practically impossible.