Why do we need to "define 12:00"? And if yes, why would that "must" be a be sun's high point? I mean, outside of the tradition and the fear of change.

We really need to finally abandon the notion of strict fixing clocks to sun. Of course without taking it into absurdity and dumb black/white pointless extremes. Semi-rigid fixing of clocks would work just fine imo. Some countries could pin sun maximum to 12:00, some to 12:30, some to 11:30 etc.

All to maximize actual insolation benefits for actual humans.

Humans > machines or ease of calculation or tradition or niche astronomy purposes etc., for me at least.