I thought I was close to a sphere, but some 1/3 center over center strokes look like they just made my oblate mirror shape less spherical. I realized I have an oblate spheroid, with a center slightly higher than it should be.
My theory is: my mirror has a slight central hill. When I press the pitch lap on top of it, the pitch forms a ring shape, with not much contact in the center. Shorter strokes without much side to side motion wear down the edge more and not the center. So I think I just need to wear down that center hill with lots of side to side motion in my strokes, placing the center of my mirror over the tool’s ring of good contact, to wear down that outer ring. Hence: lots of side to side motion while grinding.