The way I would do it is as I described, using a redirect, because it's simpler and you're also not serving the same content at 2 different domains.
I forgot WordOps actually has an alias
option for the redirect approach (not used it, but presumably will work main domain > subdomain too):
https://docs.wordops.net/commands/site/#alias-site
But yes you can do it without a redirect, it's just not a WordOps feature. You can edit the sites server block to serve both, e.g.:
server {
server_name subdomain.domain.com domain.com;
...
I think you'll need a wildcard SSL on subdomain to also cover the root domain.
You may also want to adjust: /etc/nginx/conf.d/force-ssl-subdomain.domain.com.conf
Obviously you need to make sure your root domain has an A record pointing at the server too.