Is this really 12-bit video ? How can you tell if you’re really producing 12-bit video? Aside from a handful of DCI projectors, few displays can actually render in 12-bit color. Even if you found a proper 12-bit display, it would be challenging to visually tell the difference between 10 and 12 bit video.
You shot your film in 4K/UHD… Finish it in 4K too! There are lots of advantage of finishing your film in 4K/UHD (versus 2k/HD). First, you can “future proof” your master so that it will be relevant longer. As for the present, you can use your 4K encodes to force higher quality online videos on Vimeo and YouTube.