[HELP] Any ideas on how to improve the quality of this video?

[lostsnowboardvideos]

You can denoise using the QTGMC deinterlacer, no need to do it again the denoise tab.

My filters for interlaced content, in this order (you can set the filter order under Vapoursynth -> Misc -> Filter Order/Queue):

QTGMC custom preset: preset very slow, TR2=3, SourceMatch=3, Lossless=2, EZDenoise=0.50, NoisePreset="Slower", Denoiser="KNLMeansCL"
CAS .7 - .9 depending on the source
DPIRDeblock 10.0
MCDegrainSharp - default
DehaloAlpha - default
YAHR - default
Frame (see screenshot)

Also, I wouldn't convert the color scale from 601 to 709. Would leave it as is.