Do you want to play files from a library of test media or watch movies?
If you want a device that plays maximum test media, including HDR10+/DolbyVision/AV1 it's probably a new-ish Android or high-end Windows device that natively supports HDCP and more codecs than an RPi5. There are few devices that can do that under Linux and none of them are supported by this distro which focusses on the upstream Linux kernel not downstream vendor/BSP kernels.
If you want to watch movies: 99% of the 4K media files in my library are 8-bit/10-bit HEVC, and some are HDR/HDR10/HLG (or have compatible fall-back to those static HDR formats) and almost all are 23.976fps (a handful might be 30fps) and 100% of them play nicely on an RPi5.
Hello Chewitt, very good answer.
Would RP5 + NAS be OK?