I have a similar use case, in that I run a small non-profit community where one of our users is maintaining the themes and design.
I do not wish to give anyone beside me and my co-owner access to private user data (e-mail addresses etc.), but I do have 4 moderators.
In order for the designer to work, I’ve had to create a copy site with no content where he is the admin, and I then copy themes and components manually. However it is not desirable since some changes requires content in order to proof.