Doesn't look too rich to me, it looks like the piston gets too hot. Too much timing advance/too low octane. If it was rich there's be more wash, there wouldn't be soo much baked on carbon. It's bakes on because the piston gets hot.
Yes Cool egts can indicate a rich mixture but if that mixture is burning too soon there is more heat being put into the piston dome as well, the engine will lack power due to pumping loss.
Pre-ignition and detonation do not neccesarilly create hotter exhaust temps, but they do skyrocket combustion temps and there is a difference between the two.
The wet plug also is not a fully reliable indicator. It may simply be too cold of a plug, sometimes it's very tough to distinguish when you have multiple variables that may be contributing.
