From a quick look at the relevant code in core, you’ll need extend the canPerform property (which determines if the agree/disagree etc. buttons should be shown) in reviewable-item with your additional logic:
If you’ve never done something like this before, then the Developing Discourse Themes & Theme Components would be a great place to get started! It covers how you can modify a core class which is what you need to do. Existing themes/components are a great resource that you can learn from as well, see the discourse-extra-group-classes component which uses the same technique that you’ll need to use (in particular the initialize-extra-group-classes.js.es6 file).
P.S. going to move these posts out of this topic since they’re not directly related to themes settings.