Ah, sorry, yes. I posted that in a few places, and I must have forgotten to update here when I got it working. The solution was to press cmd+f to switch to windowed mode and change the resolution.
It's not perfect though - for some reason the Mac version of the game will only support up to half of native resolution, and set at the 1080p resolution that should fit exactly below the notch the black screen issue returns. Setting the resolution to 1728x1117 does work, but it has to be scaled down by the OS to fit under the notch, so you get narrow black bars down each side.
With that combined with the capes bug, that doesn't look like it was ever fixed, I decided it was better to play in Parallels and have perfect full native 2160p.
I'll get to Deadfire once I'm through Pillars 1!