I think our concepts of “well designed buttons” align based on what I’m seeing from your examples. :)
We’re hiring designers at Booking.com if you’re ever interested in coming to test some of your hypotheses like this one. I’ve had the experience to AB test concepts like this, and have started to see things in a less black-and-white manner because of that experience. Everyone brings their own observations and experience to the table. Thanks for sharing your thoughts. It was a interesting read. Cheers!