I've known people who (typically) transitioned from being a full-time employee to part-time. Though at that point, they're usually classified as a contractor which tends to have implications for bonuses, RSUs, etc.

I agree with your assessment of consulting. I'd add that, except under special conditions, you pretty much have to keep your hand in or you lose touch with current practices etc. Yes, there are people who are real experts in some specialty. They can semi-retire and "parachute in" for a week to solve some problem that no one else can. But that is, as I say, pretty much the definition of a special case.