I couldn't compile libopencolorio and lcms2; ./configure doesn't recognize them as valid. Not even when using --with instead of --enable. I already have the packages in Arch because they're installed as dependencies for other programs.

Your new ZScale configuration also works when you change the output parameters, but the screen goes black if you touch the input parameters... F_colorspace, on the other hand, works without any issues.

I tried loading ProPhotoRGB and sRGB images in Kdenlive, GIMP, and Inkscape. Just like in CinGG, I don’t see any differences. However, in Gwenview (KDE’s image viewer) and Krita, I can see the difference. I’d say my assumption that CinGG reduces any color space to sRGB isn’t valid. As you said, there may be other reasons..

PS: I think my monitor is actually DCI P3 (98%) and not Adobe P3. It should also be 85% Rec.2020. https://www.displayspecifications.com/en/model/43583a9