Thank you very much for the explanation, and for the 2 Slide Deck presets. I took some good ideas from them to use in my ffmpeg batch files also.
This is great news! Thank you very much. 
I did a little test yesterday where I tried converting a video using ffmpeg (latest version) with libx264rgb, and noticed major glitches in the output similar to what I was getting with Shotcut… But then I tried an older version of ffmpeg (2.8 from 2015) and the conversion completed successfully with no glitches, so maybe the issue originally came from ffmpeg? Just sharing this information here in case it’s useful to anyone, the command I used was this:
ffmpeg -i input.mkv -c:v libx264rgb -preset veryslow -crf 0 output.mp4