Moin,
I'm new on this platform. The reason to subscribe were these two points:
1) I would say thank you for all the Kodi and Libreelec stuff!! I use Kodi since some days (on a rasp pi 4) that's mostly great! 
2) But: One of the things I am missing is the second display!
I'm a bit surprised that you more or less see no need for that!
I think every media center / player / etc. should have a (touch) display to operate it without TV. Every DVD player have a display!! So I think it is OBVIOUS that such a second display makes some sense!
I would prefer to have a small touchscreen for the Kodi GUI and (of course) the TV as the second for the video/movie stuff!
If I hear radio or MP3, I would control the raspi with that (always on) panel. I know I can use my tablet (an I do) but I think such a small touch display would make "a real", easy to use "product" out of my nerdy media center!
The second display could be configurable:
1) always mirror the first
2) always Kodi GUI
3) off
4) expand?
So, I felt a real need to add that here! 
Cheers
Kaktus