Multisite does no good for a test server. If you upgrade to test if there’s a broken plugin, both sites are hosed.
Same server behind reverse proxy is OK, but lots of trouble; it it’s not hard for you, then one of the “multiple discourse instances” solutions could be good for you. The easiest is a separate server and have them both share an S3 backup bucket so that it’s easy to restore data from the production site to the dev site to see how things look. That’ll also convince you that you can crank up a new server with the most recent backup.