i've noticed several posts like "depression post FIRE" and "finding meaning post FIRE".
This was the subject of my uncle's research that led to the book
The Retirement Maze, which I found interesting, but I also knew 2 of the subjects in the case studies.
At 51 he retired. After a brief honeymoon period he was fairly lost, nearly got divorced, and (per a story he told me) ordered a martini with a lot of olives, and after the bartender said, "that bad, huh?" he told her that no one has it better than him, which kind of woke him up to the fact that he was (A) very unhappy, and (B) had no reason to be so unhappy - healthy kids, more than enough money, a partner who he liked and trusted.
So he funded what became the largest independent study on retirees, used his skills as a research psychologist to interpret the data, and with the help of his old thesis advisor and a business partner, wrote the book, which led to 2 other books.
17 years later I would like to think he did a great job in his 2nd act, and as he nears 70 is successfully entering a 3rd.