Can anyone confirm that a lens' sweet spot is typically 2 stops closed from the lens' maximum aperture? IOW, if my max is f/2.8, f/5.6 is at least a starting point to test for the lens' sweet spot. Comments?
Completely depends on the lens, and your definition of "sweet spot".
If by "sweet spot" you mean the best possible sharpness, then for many lenses it's usually around 2-3 stops away from widest. If you're not sure, set the aperture to f/8 and you'll likely be near the sweet spot.
My 18-135 IS STM is very good wide open, excellent at 5.6.
My 50 f/1.8 is quite good at 2.2, excellent at 2.8.
My 18-50 f/2.8 Sigma isn't amazing at any aperture, it's as good at f/2.8 then at any other aperture.
My 8mm f/3.5 is decent at 3.5, amazing at 5.6.
Note that these are findings I've made with my particular lens, your copy might be different. I've seen alot of variation in lenses over the years.
First thing I do with a new lens (or when trying out a used one) is take a bunch of comparison shots are various zoom levels. I always considered my 55-250 a pretty poor lens, and wide open it was quite soft. But one stop from wide open that thing sharpens up a ton. At 250mm f/5.6 it's soft, and f/8 it's very good, f/11 doesn't seem to help it more.