* Update * Solved, I can build and run my standalone Apple Watch App again.
I've cleared all simulators from the list click on 'Apple Watch Series # (In the top bar next to the Play and Stop Button, next to AppName > Device Simulator Name) and click 'Add additional Simulators'. Click the simulators you're using and hit '-'. I also deleted the iPhone Simulators on the left (right click, 'Delete'). Then add a new simulator by hitting the '+' in the bottom left corner.
I then set the Target (Hit project name with Blue Icon at the top of the left section), then to the right side select the Targets. There should be 3 targets visible for an Apple Watch Standalone App: 'AppName' (set Build Settings > Supported Platforms set 'Base SDK' and 'Supported Platforms' to iOS. Then for the 'AppName App' and 'AppName Extension' both the 'Base SDK' and the 'Supported Platforms' should be set to 'watchOS'.
Not sure whether it was my simulator references being outdated or that changing the Base SDK and Supported Platforms was the issue (maybe both) but I hope this helps anyone who ran into the same issue.