I subbed a few times a month last year, and my experience in it was similar to yours. You shared helpful points, and are completely right that a lesson plan with lots of “read silently” isn’t going to go over well.
Personally, my biggest problem with subbing connects back to the piece I read today about being useful. I too often felt like I was only expected to babysit, not teach, and I hated that. I appreciates teachers who trusted me, the lowly sub, with a lesson plan that included an actual lesson!
Thanks for sharing your experience, very helpful to someone wondering what to expect.