@denny2k2 I found your idea to have a heimdall dashboard as a starting page intriguing. So i tried to set it up myself and first hit the same point as you. In my case I solved the problem of the "bad gateway" endpoint by joining the letsencrypt and heimdall docker in a network. My letsencrypt docker was already in a network called my-net together with the nextcloud docker (following TechnoDadLife's tutorials). So I added the extra argument --network my-net to the heimdall docker. After that i could reach heimdall over a subdomain of my duckdns account -> e.g. https://heimdall.mydomain.duckdns.org
You sly dog!
That was actually on my "to try" list.
Glad that it worked for you. Will give it a bash myself.