Correct me if im wrong, but if you play a 1080p video on a 4k screen, that would be upscaled. If you put a 1080p video in a 4K stream, then play that 4k stream on the 4k screen, no post-processing would be applied to the video on the screen. All the upscaling happens during encoding, where you have far more control over the upscaler quality.
Both UnraidFS and mergerFS can merge drives of separate types and sizes into one array. They also allow removing / adding drives without disturbing the array. None of this is possible with traditional RAID (or at least not without a significant time sink for re-making the array), no matter the type of RAID you use.