Sorry but your argument is moot. Yes NAT is not firewall.
No my devices are not receiving random packets with internal IP addresses often enough for me to worry about it.
Even better not single of my devices living behind only nat in 20+ years was taken over or ransomed.
If it would be real problem I would hear about it daily and - if it would be so easy - all kinds of APT groups would take over devices behind NAT en mass. I don’t believe that any residential router has firewall enabled by default and only devices that are taken over are ones really having public IP.
Anyone can prove me wrong sharing links to articles and instances where there was occurrence of mass takeovers using NAT traversals.