Hi Dan,
I would highly recommend having a look at App Designer (included by default with MATLAB, no additional licenses required) for the creation of custom User Interfaces to control your real-time application. This tool offers much more flexibility than Simulink Real-Time Explorer and enables you to design both the front-end (GUI) and the back-end (code). It recognizes all standard commands from the Simulink Real-Time API (see here for R2020b and later / here for R2020a and earlier), which can be integrated in the app’s back-end (code). You can get more information about MATLAB App Designer and try several examples here.
Best,
Dimitri