that is why i stopped using ProRes, I noticed it has weird quality issues on my Video8 processing in some cases (i posted a whole new post about this issue here in the forum).
now I only use the lossless x264 and the lossless FFV1 (started to like that one). I personally couldn’t tell the difference in quality between the two, to me they look the same.
once you de-interlace your videos, check out @TomaszW presets for upscaling.
I personally found two that works great for my Video8 files.