It's like I could've written this comment. I'm working on 4 days per week, and I agree with everything you just said. I've worked on both 5 and 4 days per week. The issue in my case with 5 is: I get insomnia issues for a multitude of reasons. I also have them with 4 days per week, but it's easier to build in fallbacks and so on. My 3rd free day is on the Wednesday for this reason.