I do understand I can make a custom page with the custom-page-plugin. I should be able to get the widget and login buttons to appear there. I'm just not sure how to make the login button go to that route.
I don't necessarily need to replace /login, I just want to make sure that clicking login or doing something requires login (such as replying to a topic) takes them to my custom login page.
Solved by changing on Nginx configuration proxy_pass :
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Forwarded-Proto https;
Today I changed the nginx config. Now I use a stream with map and a generic server in nginx.conf (see TLS Termination). This implied removing ssl and http2 from the server's listen directive in sites-enabled/my-site.
listen 127.0.0.1:445 deferred ssl http2;
listen 127.0.0.1:445 deferred;
That's about my guess why https is required for proxy_set_header X-Forwarded-Proto now.