First let me thank you for that great blog post!
Over the weekend I tried to follow you guide but I came across a curious, at least for me, problem. Ip forwarding seems only to work if I start the network service after the shorewall service. On the clients when I try to traceroute any webaddress, im stuck at the router, after stopping and starting systemd-networkd.service the route completes. Is this normal behaviour and I just did something wrong, or did you take care that the network service starts after the shorewall?