How do you imagine that working? An admin has access to everything, but maybe you wouldn’t like it if other testers had access to your email address and your IP address. In order to try things out, you would also have to be allowed to change things. How do you ensure that this isn’t exploited, for example, that no one installs a theme component that spies on others?
It’s quite easy to set up a dev install in a GitHub codespace. I have used that for testing the admin experience before: Developing Discourse on GitHub Codespaces. Of course, any other dev-install works too.