I think you're conflating firewalls and NAT. The idea of "internal" and "external" networks still apply in a non-NAT environment but what takes some getting use

I think you're conflating firewalls and NAT. The idea of "internal" and "external" networks still apply in a non-NAT environment but what takes some getting used to is that with full end-to-end connectivity, you're back to an implicit "default-allow" policy where NAT created an implicit "default deny". The answer is to have a default deny firewall rule on your border router (your home gateway appliance), and then allow services as needed.