>I think a 3rd or 4th burnout at 30 years of service is a lot harder to recover from, but I’m biased from my own experience

There is something else going here (or so it was for me). It's not the physical/mental exhaustion as is typically what defines 'burnout'. It's the depressing realization that most people around you are deluded piles of shit, even if they are 'nice' individuals.