Display fn keys by default in Touch Bar
In System Preferences → Keyboard → Shortcuts, under Function Keys, add apps to the list to show the function keys by default. The fn key will then be used for the inverse function, hiding the function row instead of showing it. You'll need to do this for every app where you would like this behaviour.

You can display function keys at all times and use Fn to display App Controls or Expand Control Strip in System Preferences → Keyboard → Keyboard.