I now know it's a one year old thread soon, but the weirdest of all about this with macOS and HDMI output being stuck at YCbCr is that the Apple TV and tvOS gives the options to change the output to RGB – why the heck is this not possible on macOS (without third party software)? I guess it is the same in macOS Tahoe (I haven't tried it yet, still on Sequoia).