Have you tried the DLSS / DLAA option.
I find this gives the best quality image on my System. The only slight shimmering I see with the DLAA option is when I’m flying the JF piper arrow, I see a little bit of shimmering when looking through the prop.
Game mode on/off won’t make any difference to the visual quality, it just basically stops some Windows background tasks from running when you are playing games.
