How to use FN keys as FN keys on macbook pro touchbar for ALL apps?
If you go to Settings > Keyboard, in the Keyboard pane, the box "Touch Bar shows" select "F1, F2, etc. Keys" as the option.

The touch bar will now just show Fn keys as if it was a normal keyboard. (They will dim and appear black sometimes.)

Then, you can change the setting directly below, "Press Fn key to" and select "Expand Control Strip" or "Show App Controls". This will fulfill your request to "only have it while I hold the fn key."

Hope this helps!