FCPX, Studio Display preset at Digital Cinema (P3-DCI), seems way too dark?

I was trying to make some print stills pulled from a current film (from the FCPX timeline). They were screamingly dark, when printed. So I reset my Studio Display preset to 'Design & Print (P3-D50)' which made the display a lot darker. But I also realised that this may be my problem with the dark prints. I had to jack my colours and brightness way up, to be able to get a decent image on the darker monitor, so I'm thinking it might work, need to find an inkjet printer to test.

Then I'm reading online advice from Google -- which says for film editing my Studio Display (5K) should be set at Digital Cinema (P3-DCI). So I set it. It's horribly dark. Even difficult to see.

A couple of years ago I colour graded several IMAX films using just my MacBook Pro Retina screen, with its factory default monitor setting, whatever it was. And an Apple Studio (not mine) that was set such that it was identical to the Retina on my Mac. And the end result was terrific in the IMAX theatres, exactly as I'd seen it on the Retina and Studio Display monitors.

Google recommends setting the Retina to a 'Display P3' ... which looks about the same as the 'Color LCD' that was the factory preset. So this at least makes sense. And I'm reassured.

But the big Studio Display is almost too dark to work with, and, with the 'Digital Cinema (P3-DCI)' preset, one can't change the brightness of the screen in the System Settings, it's locked, greyed out.

Anyone got any ideas about this? I was wanting to use the Studio Display to colour grade a current 4K film that's going to be running in IMAX theatres. Now I'm super confused.