Set F1...F12 as default on app-specific basis?

I'd love to set Esc F1...F12 as default for Xquartz (normally, I'd have to press fn key for that; but for Xquartz, I'd like those to be the default and to press fn for the usual).

Is that possible, preferably without having to create my own F1...F12 icons and button group from scratch?

You don't need BTT for this at all. Go to System Preferences > Keyboard > Shortcuts > Function Keys. Add whichever applications you'd like to the list there and it will show the F keys instead of the normal touch bar.

Thanks, that's what I wanted!