Also, is there a GPU that will hardware-decode the video I attached, and if so, which one ? The clip is H.265 8K 24p, vertically shot, from a smartphone - probably variable bit rate.
Answering my own question here. I went back to my HTPC which has a GTX 1050Ti GPU. Looks like I had disabled hardware decoding in MC x64.
As I recall, it's because of issues when playing back UHD Blu-Rays discs. When moving the mouse and having OSD overlay, it causes the video mode to switch. The projector is very slow with mode switches (about 5 seconds). By the time they are displayed after a mouse motion, MC is ready to switch the OSD off due to timeout, and goes back to original video mode, without the OSD.
I switched HW decoding back on, and the 8K / 24 vertical clip played fine in MC, utilizing 5% CPU only, and 65% on GPU video decode. That is nice.
But I would rather keep HW decoding off, due to the mode switch issues with the projector.
And the CPU in that machine (Ryzen 2700, no OC there) is powerful enough to do 8K software decode in Windows Movies & TV. The problem is with the 8K software decode in MC.