Set your desktop resolution to 1920x1080 + find the option in your monitor's menu that disables upscaling to its native 2k resolution. This means you'll play with a much smaller window of course. There's not really anything else you can do until Stadia supports more resolutions.
I also have the same problem on the 4k screen, as soon as I start the game it looks good but the quality drops instantly. If anyone knows any solution to tell us!
I'm just waiting for good to update and fix it no way am I going to jack up my high quality for a single game, they really need to make the resolution options more extensive for PC users.