I was having the webfinger issue with a similar-ish setup, and what worked for me (for the webfinger warning at least, hopefully the other well-known warnings too) was deleting the nginx default.conf and letting it regenerate after restarting nextcloud (`/config/nginx/site-confs/default.conf`).
I also had to clear cache since the 301 response gets cached and so the warning won't clear. You need to Open Dev Tools (F12), and while this is open right click on refresh button and select Empty cache and hard reload.
More context here: https://github.com/linuxserver/docker-nextcloud/issues/189
I realize you might not be using the linuxserver.io nexcloud docker image, but I hope this helps!
Edit:
To fix the "Strict-Transport-Security" HTTP header warning, add these lines to your `default.conf` (`/config/nginx/site-confs/default.conf`):
# Add headers to serve security related headers
# Before enabling Strict-Transport-Security headers please read into this
# topic first.
add_header Strict-Transport-Security "max-age=15552000; includeSubDomains; preload;" always;
#
# WARNING: Only add the preload option once you read about
# the consequences in https://hstspreload.org/. This option
# will add the domain to a hardcoded list that is shipped
# in all major browsers and getting removed from this list
# could take several months.
You'll want to do this after regenerating `default.conf` or you'll have to re-add the lines.
