>Also, slightly contradictarily, natural selection/evolution don't provide much pressure for a species to change after it's gotten wildly successful.
It provides pressure for the species to change to adapt to its wild success (e.g. to become "fat and lazy"). That development of the wild success may also lead to significantly changed conditions (e.g. global warming as a result of the wild success of homo sapiens)
>Have we had any physiological brain changes since agriculture?
it is only 10000 years, and yes, we had. The physiological changes are observable even for the last 100 years.
>And now that medicine is becoming so effective, natural selection doesn't really work on the fitness of people, just their ability to have more children.
that phrase sounds like oxymoron.
Anyway, natural selection is like Newton's laws - it can't be turned off. It works non-stop. It ensures fitness to whatever current conditions are. In particular, right now, it ensures fitness of human species to the condition of "medicine is becoming so effective".