A view showing some points being pulled to the left, as seen on the right.

More edge-insertions and deletion steps below

At this stage I felt dissatisfied with the SubD version of the foot - it was still too smooth, and lacked detail.

One of the most obvious characteristics of the foot is it has much more detail than many other parts of the body. The contrast is especially apparent on women. It's also true of course for hands, knees, elbows and central/lower front of throat etc, though usually less so.
So I went back to the reference and did another round of sketches. On the left I've highlighted 'knobs' of bone and muscle that are visible most of the time, on most feet. On the right I've underlined the drooping shape of the ankle bone. Another thing I noted, which was unexpected, is that on some women the first knuckle of the index-toe - it's 'base' - is lower than that of the little toe.

With those knobs added:

And the final topology:

Select these edges, then do a Subdiv Surface > Full Crease.