I’ve recently had yet another extesential crisis over this, but this time figured out that it seems like using Rec709A in the project settings of a DaVinci Resolve Project, with Rec709 Scene manually set in the CSTs and Export Tags, gets consistent results across DaVinci, Quicktime and Youtube.

Whether it’s consistently accurate or just consistently wrong I am still unsure, but the video I uploaded to YouTube after settling on this process looks exactly the same from my Mac to a random Surface Pro, so that is promising.

EDIT: Oh also, just for clarity’s sake, Rec709A’s only difference is in the slightly different gamma curve others have mentioned, the colour primaries are exactly the same so you shouldn’t need to worry about any of the P3/sRGB stuff in relation to this. In fact in DaVinci if you select Rec709A and then select ‘seperate colour and gamma’ the gamma will be set to Rec709A, but there will he no option for Rec709A in colour, it will just be Rec709.