I am in a similar financial situation and age ($6M liquid, $60k spend, 38, not single), and have been in burnout for a while and haven’t found the courage to quit yet.
Similar burnout reasons as yours: nothing is horrible about the work but I feel deeply inadequate, I feel in my 20s I was a high performer with incredible mastery of my domain, and as the industry “progressed” complexity-wise and everyone became more advanced in their craft, I got left behind. There are many (most?) younger (and older) folks at my current company who are producing 100X what I do (this is not some kind of impostor syndrome btw, I literally had a trusted colleague say to me: “you really should be more ambitious and build more, much more”), and are hired on the same ladder, for the same comp. As you stated, that chips away at your confidence every day and it plants the seed for burnout.
Similar worries as yours: I am quite honestly afraid of what I will do with my time. I would just be running away from work and not running towards something else, and having grown up with some money trauma I did not want to regret willingly stepping away from the high pay, but I think a forced layoff would probably be the best gift someone could give me right now.
I hope for many more insightful replies to this thread.