According to the WidgetKit docs for Lock Screen Widgets and Watch Complications, the recommended way to scale text within a widget is to use the sizeThatFits API ("Use SwiftUI’s sizeThatFits(_:) API to scale text so that it fits available space.")
I can't find any examples of sizeThatFits being used this way, so I don't really understand how to implement this...
So far I am just using minimumScaleFactor, which works fine, so I'm not sure what I'm missing here.