Not really sure everything in this list is needed, but this is working on my 2017 15 inch macbook pro running Catalina 10.15.1, with an external 4k monitor...
1. Get rid of all spaces (the additional swipe/slide screens on the mac) but one.
2. WIth Duel Screen off, set Workplace / Primary Screen to Screen 1
3. Shut mac down.
4. Turn off external monitor.
5. Power up mac.
6. Open DaVinci, which should display fine on your mac.
7. Power up external monitor.
8. Hit "Screen 2" under Workplace/Primary Screen.
9. Throw salt over your shoulder if needed
Tried turning duel screen on before and everything disappeared, so, very content at the moment to have DaVinci consistently visible on my larger monitor!
Cheers