Instead of showing two fixed modes (desktop vs mobile) implement possibility of dragging edges of your templates (in both the editing and preview mode) to see how they look on different screens. Also additionally to custom resizing allow choosing different predefined sizes based on device viewports (e. g. Desktop, iPad, iPhone 12 mini etc.)

Similar function is available:

– Web: Webflow, Framer

– Email: Parcel, Tabular