[brazen1]

Ok, so sorry if I've missundercept something regarding the actual complains, or if i would be somewhat inexact or mistaken in these technical phrases and specific options,  specially regarding NVidia-side possibilities, but I have to add:

That's because Windows is only compliant at 8bit when anything higher than 30Hz.

-> Are you sure that's some PC/Windows standard regulation and not just caused by the (buggy) NVidia drivers? See: https://www.extron.com/company/article.aspx?id=hdmi2faq
Yes, I'm sure.  When using RGB Full (4:4:4) the best you're ever going to do is 8bit at anything over 30Hz.  Your link clearly shows that.  Has nothing to do with your blame that this limitation is due to buggy nVidia drivers?  It's the specification standard.

NCP will show 12bit but later in the processing this will dither down accordingly.

-> "Dither" to what, 10bit or 8bit? Thought it was explained clearly?  When set to 12bit, dithering to render what it is.  SDR is 8bit.  HDR is 10bit.  Cause 10 bit -may- just would be fine, but as it's stated as "dither" and as preferably it would be better to stick to stock 10bit anyways, i guess what you name "dithering" should evolve some post-processing resulting a not-so-much-native PQ. Am i right on this one?  I don't understand what you mean here?  So the 10 bit -even- in this case would be much better than 8bit, but why not just set the native output to 10 bit as the wast majority of the 4K sources jet would make it self-evident as they are also mostly 10bit.  Explain to me how to set 10bit?  As I stated, nVidia has no 10bit setting when using RGB 4:4:4.  Only 8/12.  This is where you can interject your dislike of nVidia and its buggy drivers.   Instead I set to 12bit which dithers down to match the bit depth of what ever source is played.  You can't magically convert an 8bit source to 10bit by using a 10/12bit setting?  It's going to dither down.  Also, while your only concern may be what you consider the vast majority is 4k 10bit sources, personally, I have many others and tailor to accommodate ALL of them especially the Windows Desktop which requires 8bit at best at 60Hz.  If you want to set your settings different because you think they work better for the limited playback you require, have at it.

I wouldn't get too hung up on bit depth.  You could be doing more harm than good by introducing banding and everything at 8bit would be fine with little to unnoticeable difference vs 10/12bit. ->

I guess you din't see much 10bit vs 8bit bitdepth PQ comparison samples. There can be a HUGE PQ effecting difference in how color banding is detailed and refined between 10 and 8bit, with much and undeniable advantage for the 10bit side (sometimes may also be dependent on the source video, but many recent true 10bit videos will show this difference undeniably even for the most sceptic or verdant typed eyes). -> This all effects of course if you have also a proper 10 bit display to the do these "final" task.  Yes, I have a native 10bit display.  A very nice one.  Yes I have many UHD HDR unaltered titles and I play them all at the maximum bit depth they were mastered in.... 10bit.  At times I prefer 8bit.  Reducing my GPU settings to output 8bit and matching that setting in madVR doesn't leave much on the table.  What it does do is lessen (not eliminate) banding so many times 8bit with less banding is preferred over what ever you think you are visibly gaining using 10bit.  A highly compressed altered and butchered rip might benefit but personally I use untouched 1:1 rips.  Not sure how you see such a phenomenal difference but you should present your findings at doom9 forum where experts will evaluate with you.  Imo, when it comes to HDR and bit depth, more concern should be placed on chroma upscaling instead.