Got it figured out. Basically, you have to form a "redirect chain" kinda. I think I was creating an infinite loop of redirection or excluding a specific case (www. for 443 and for 80 separately) in my above nginx config. I ended up having to redirect 80 http://www.domain.com, 80 http://domain.com AND 443 https://www.domain.com all to 443 https://domain.com and then that third block is where all the master logic for the activity goes, if that makes sense.
In my original post, I was combining too many of these cases into just 2 blocks. In addition, I also added location / {} and wrapped that around my return 301 statements.
This fixed the problem entirely with both the domain and the forum malfunctioning. Last but not least, I wanted to mention that I also added proxy_set_header X-Forwarded-Proto $scheme; to the main logic block near the other proxy_set settings, which fixed a csrf error as well. So we're all good now!
Very exciting stuff. I appreciate the quick replies and help. Happy to use NodeBB for my new site!