If sensitive to flicker and wanting to avoid it, would the Vizio M be better, as it appears to be flicker free at max backlight level, or would the Sony X900F be a better choice due to its overall higher flicker score?
Image quality from a composite source is always going to be terrible. Try vivid picture mode instead or use standard and tweak to your own liking.
You should be using HDMI if the set top box supports it.
If this is an old set top box with only analogue video output, check to see if it has s-video or component outputs which are better quality than composite then get a s-video/component to HDMI converter and feed that into the TV.