[yummyfajitas]

Do you have any idea how much time teachers spend grading and preparing lessons after the school day ends?

No, the data I gave only lists time spent working (at home and at school), and does not break work down into specific tasks. So overall, I know teachers work 2.5 hours/week less than other professionals, but I don't know what they are doing while working. Why does this matters?

...that's a boatload of material to edit/revise over the summer.

For new teachers, sure. For experienced teachers, not so much. My first time teaching calculus was a lot of work, my second time it was pretty easy.

As for the customer-facing nature of teaching, I agree that it's not for everyone. I certainly didn't like it, but some people love being the center of attention. If you don't like customer facing work, find a different career. Don't expect brownie points for sticking it out in a career you are poorly suited for.