Instead of writing iptables by hand, maybe this would work: https://fedoramagazine.org/internet-connection-sharing-networkmanager/ ?
Good Job Purism !
I guess that would need to be tested on the Librem 5.
If it does work then it avoids the need to assign static IP addresses on the ethernet side and instead gives you DHCP for the clients.
I wish that had come up in testing.
I updated the video to show using networkmanager instead of iptables.