I'm guessing that you've probably already made a decision and/or found another solution, but I'll answer anyway, just in case...

We do not provide any "ready to go" reverse proxy appliance, but you could use either our Nginx appliance or our LAMP appliance as a reverse proxy (they perhaps obviously include Nginx or Apache respectively). Having said that, both have lots of additional stuff that you likely won't want or need in a reverse proxy (MySQL/MariaDB and PHP for starters). So another alternative would be to start with our Core; our base appliance. Then install the desired software (I'd recommend using apt to install from the Debian repos where possible - that way you'll get auto security updates for free).

Unfortunately, OOTB non commandline UI options are a bit limited. The LAMP appliance includes an Apache Webmin module, but we don't have anything for Nginx. Regarding the Webmin Apache module, if you start with Core and want that, you'll need to install via apt and restart Webmin. I.e. (as root):

apt update
apt install webmin-apache
systemctl restart webmin

FWIW, I personally prefer config via commandline. As a general rule, it's much more verbose and provides much more useful feedback. I find Apache CLI config pretty straight forward and via Webmin a bit confusing (but obviously YMMV). You'll also find tons of Linux tutorials online that will assist you to get up to speed with CLI (command line interface).

If you'd rather go with Nginx, then I'd definitely recommend CLI. You could install a web UI (i.e. web based GUI) yourself. But as you'll need to do that via CLI anyway, and reverse proxy config is fairly straight forward, unless you plan to constantly tweak your config, it's probably easier to just set it up Niginx via CLI.

Regardless, if you keep in mind that TurnKey is based on Debian (v16.x = Debian 10/Buster) then you should find plenty of relevant info and tutorials online. Plus I generally reply here a bit quicker than I have this time... :)