I think the most effective way to learn is to stay at a place long enough so that you are still there when (not if) your decisions a couple of years ago come back to bite you. You need to experience full feedback cycles to become a great engineer and that can take 2-3 years after first shipping code to become apparent. I never had a mentor either, but I stayed at my first job for 11 years and I would never replace that experience with a mentor.
Many of us are clever enough to keep the wheels on pretty much any strategy for about 18 months. Including process. I think the industry was done considerable injury not by developers job hopping every two years - although that’s also bad - but by managers doing so.
