I have photo set to auto. Also I have motion photo on and I have it set to 3:4 at 108MP.
(S21 ultra 5g unlocked)
When you adjust your shutter faster the screen will darken so at that point you will need to increase your iso. There is a compromise in that the faster the iso the more noise in the photo but I prefer an in focus shot with noise than a blurry shot without noise.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)