So generally, I’ve always understood that the lowest ISO is best to shoot at. Though with newer cameras, they usually have a higher base ISO and in some cases Dual ISO.
I usually work with the R5C, even for photography. It feels odd to take portraits at 800 ISO because I’ve always been told it should be as low as possible.
So does the Base ISO system, negate the need to shoot at lowest ISO for the clearest and least grainy image?
If you don’t need to under expose, then shooting at the lowest ISO will give you the least noise.
Noise in photos is mostly from the light itself. But some of it is from the Camera Electronics, and when you bring up the exposure in post, you are bringing up that noise too.
A perfect camera could be shot at base ISO, under exposed any amount, and adjusted in post with no harm. Many cameras are pretty close to that over a limited range but not the entire range.
So for your example, if you have to choose to Expose at ISO 100 and under expose by 3 stops, or shoot at ISO 800, you will have less noise in the ISO 800 shot.
If you do not need to under expose the ISO 100 shot, you will have less noise in the ISO 100 shot then the ISO 800 shot.