I’m a Knox grad, and I somewhat touched on this question in this thread: http://talk.collegeconfidential.com/knox-college/2130411-rd-2019.html
You might also reach out to @Theoden whose son is a current Knox student.
Mostly, the schools you’ve listed will offer rigorous academics, yet the stress issue should be not super intense. I’ve heard (I could be wrong, though) that Guilford is a bit too hippie-dippy and that its academics could be more challenging. But I’m sure that for the dedicated student, rigor can still be found there. Again, I could be wrong, so it would be nice for a Guilford student or parent to correct me.