I gave up on trying to gauge the outcome of the interviews. Most of them I thought I did very well on, some above average, and a few I knew went poorly. All of them had the same outcome.

One interview battery in particular stands out. I interviewed at least 6 times over two weeks, some 1:1, some 1:many. I interviewed with the team, an unrelated employee for a culture interview, the VP of engineering, and a C level exec. All felt great, all feedback from the recruiter was positive, but after everything was done I was told the position "wouldn't challenge me enough" or something similar. I was devastated.

I have never thought about asking for interview feedback during the interview. That is a good idea.