@misterb Awesome find ... it's very subtly snuck in there!
I ended up importing the Let's Encrypt CA, generating some certs, and ultimately once you get RESTCONF up and running, the Web GUI will become available. Generally, following these 2 guides will get you up and running:https://docs.netgate.com/tnsr/en/latest/recipes/restconf-pki-nacm/index.html https://docs.netgate.com/tnsr/en/latest/recipes/gui/index.html
Admittedly, if you run with a Public CA on the box, you'll have to do a bit more import and signing work, but it's not that bad. At least we figured out how to get the Web GUI running.
I'm having an issue getting the Client Cert imported on my mac (it claims "password is incorrect" despite export via the TNSR CLI and openssl command), but that's a separate issue. And also, you could try to swap client-cert auth for password-auth, but that's not as secure so I'll just figure out the Client Cert issue.