I believe the right *mix of solutions was as follows:
As per @itsbhanusharma:
EDIT /var/discourse/containers/app.yml and amend ports to some custom, I used:
- "8080:80" #http
- "4343:443" #https
Did a ./launcher rebuild app
I then modified our externally accessible proxy to forward requests on 80/443 to http://internal_ip:8080
After a sudo nginx -t and sudo systemctl restart nginx I logged into https://discourse.mobiusnode.io server (which was still exhibiting the issues above) and enabled force_https, at which point it appears all is working.
I am now going to reproduce this on the remaining servers/infrastructures.