If you haven't already, have you tried reset world?
I've tried resetworld. I've tried fixall. I've tried resetting every single sim in the household individually. I've tried without CC. I've tried moving the family. I've tried moving the lot. I've even tried turning my fucking computer off and on again.
Moving the household to a new clean neighbourhood seems to have worked, but I fucking hate doing that. It takes forever to get things back to the way I like them.
Edit: The everlasting pool party is gone, but my ultimate sim has gone invisible/stuck underground and none of the usual tricks work.
I suspect that the game just can't handle an immortal sim trying to excel at every skill, job and profession in the game.