Just in case anyone else runs into a similar problem, I was looking at the wrong api urls to get what I needed.
I needed to be getting JSON from the topic data, rather than category or post. Most everything is working now.
I was trying to switch to Apache instead of Nginx and I did all the instructions on https://docs.nodebb.org/configuring/proxies/apache/
But when I visit forums.mydomain.com, the default Apache welcome page appears.
I’d appreciate any help!