I don’t know what source you are using, but my flow for my source videos is run a Nyx v1 (Auto) clean up (keeping it at the same resolution), because mines a bit noisy. then upscale x2 using Dione TV.
For me it comes out sharp relative clean of artifacts, much details preserved & it keeps the original looks of the video, just in HD resolution.
I know @ssbroly did extensive testing to the border line of obsession (in a positive way) for it to become perfect , so maybe he has better method that might suite you better.
Also @ForSerious developed good results and has some workflow(s), maybe he can share as well.
I am sure there are others here as well, but those are the two that i was more exposed to on upscaling workflows.
P.S. yeah, going from SD → 4K might be a disappointment, I would stick with SD → HD until you get good experience with the tool at least…
