I don’t know why I can’t edit my previous post, but moderator may merge them.
I just wanted to add that I think my problem and solution MIGHT have been that I was too eager.
Because of having installed a faulty plugin, I recently had to scratch and reinstall my discourse instance. When I started this new instance, I got a new IP. So I went to my hosting service, updated my A-pointer/DNS, which points my subdomain discourse.mydomain.com to the IP of my Google Cloud instance, and then I went back to my Google Cloud SSH to run the setup.
When you run ./discourse-setup the first time, the setup checks the ability to bind to ports 80 and 443. But for some reason, it once again refused to bind to 80 and 443. I looked up the DNS by who.is and the subdomain resolved fine to the IP and all - but it still didn’t work. Tried running ./discourse-setup 3 times, no luck. Checked with netstat -ltnpu, nothing special. Waited 5 more minutes, ran it a fourth time, and now it worked.