You can use Chrome extensions to modify Stadia, I used Stadia+[0]. You can force 4k that way even though it's not supported.
I don't have any noticable input lag but I'm on 70/30mbit fiber with 3-4ms ping to stadia.com. Sometimes the stream quality drops for a sec, but no noticeable frames or audio dropped.
I agree with the darkness being to bright, couldn't fix it with the gamme correction either. I have a feeling the 4k does a little better job in that regard. But overal it's not bothering me to much.
[0] https://chrome.google.com/webstore/detail/stadia%20-extensio...