So in my niginx.conf, in my two server {} blocks (one HTTP->HTTPS redirect, other HTTPS)
I had to set `server_name h2ochan.org www.h2ochan.org;` - I mistakenly thought that having two server {} blocks with identical (or multiple!!) server-names would be bad, but it is fine.
I also had to go to my DNS settings, where I had an A/AAAA record with hostname "h2ochan.org" pointing at my server's IP address. I tried adding "www.h2ochan.org" at the same IP, which simplified to hostname "www"
It could have taken 72 hours to propagate, but seems to have worked within 5 minutes, and now my HN link (https://www) works instead of saying "page not found"!
h2ochan.org works fine, redirecting to https://h2ochan.org/