[noggin]

I guess it should auto detect the source and be able to switch to 444 / 10bit for <30Hz and 422 / 12 bit for >30Hz. Or possibly could 422 / 12 bit also with <30Hz make more sense? I've read a couple of times that 12bit output can make the color transition a bit better, even if both the source video and the TV is still just 8 or 10bit (just like in the case of 10bit compared to 8). Or the "closer" Chroma 444 setting would have more positive effect on the overall PQ, than the benefit of 10->12bit "smoothening"?

Hmm - I guess some displays will do different processing on a UHD signal with 10-bit content presented as 10-bit (at 4:4:4 for <=30Hz or 4:2:0 for >30Hz) vs 10-bit content presented as 12-bit (at 4:2:2 for any frame rate or 4:2:0 for >30Hz)?  

However some displays seem not to like 4:4:4 10-bit content at any UHD frame rate, whilst others only accept 4:2:0 10-bit at higher (50/59.94/60Hz) frame rates.  (Many Sources, AVRs and TVs have options that enable 'Enhanced' modes for UHD HDR capable HDMI inputs/outputs that mean switching to 4:2:2 12-bit from 4:2:0 10-bit I think. By default these are usually disabled - meaning 4:2:0 10-bit is the default choice?)

I'm guessing the safest combination if you can't do full EDID-based output selection may be 4:2:2 12-bit (carrying 10-bit) for <=30Hz and 4:2:0 10-bit for >30Hz as these are the lowest bandwidth outputs available that won't truncate bit depth?  I may - of course - be very wrong.