How to render png's as h.265 12 bit video?
Use

ffmpeg -framerate 500/21 -i %d.png -i audio.wav -c:v libx265 -x265-params lossless=1 -pix_fmt yuv422p12le -c:a copy output.mkv


You'll need a ffmpg with a libx265 that supports a bit depth of 12. At this time, I can't find such a precompiled binary so you'll have to compile ffmpeg using the auto-build script at https://github.com/jb-alvarado/media-autobuild_suite