> If you assume that having the appropriate firewall rule to do that is the default
That's the thing, it's not the default, default is public ipv6 for everyone and its the users duty to configure firewall...
I could definitely set this up easily, someone like my parents or friends would ask me 'what's IPv6?'
Ah, okay. In that case v4 doesn't have a firewall by default either.
That's precisely why routers come configured with a firewall that blocks inbound connections from the WAN -- because the protocol itself doesn't have a firewall by default, and neither does NAT.
NAT contributes nothing to your security in this scenario, and instead makes it harder (not easier) to understand and reason about what your router is doing.