OLED Motion Pro in my experience isn't ideal for 30fps games - I usually only use it on 60fps games. However you may perceive it as sharper in motion even at 30fps, but for me the motion looks very strange with OLED Motion Pro in 30fps games. People see these things a bit differently!

Auto should in theory work with Switch unless another device in your video chain is throwing it off. I would try setting both the TV and Switch to Limited and then do the same for Full. These scenarios should have identical contrast/gamma/color injured there's some other configuration error. If you have a mismatch between these settings, it's possible you could end up blowing out highlight detail and crushing shadow detail. This could create a perception of a higher contrast image and look "better" at a quick glance, but it would be incorrect technically and would make things even more difficult to see in dark areas. You likely would not have seen true black at all if your settings were wrong in the opposite way - the underground with no light sources would have gray instead of black. This makes me wonder if your previous automatic settings were correct, and you just didn't care for the game's pastel colored grass or something. Or if there was another issue with your settings. Hard to say really.

I dunno, I honestly thought Tears of the Kingdom looked awesome on my OLED (TV and Switch). The resolution may be low, but it was fine enough for me, and the use of color and general atmosphere is excellent, so it was still very pleasing to my eyes.