I can suggest an Intel Gemini Lake ITX board + a small ITX case + a cheap (used) DDR4 SO-DIMM RAM stick (2Gb will be fine). Should be cheaper than NUC.
It does play H.264 2160p60 - I just tested with "Sony Sushi 4k Demo" clip, it plays it using VAAPI hw acceleration. HDR support is also on it's way for Gemini Lake (it will be supported in Linux kernel 5.3)