Agriculture appears to have been developed in southwest Asia, probably Mesopotamia. It would not be unlikely for Caucasian agriculturalists to spread out from there, across the Fertile Crescent, down the Levant, and down into Egypt and North Africa, displacing any black hunter/gatherer inhabitants who might have been there earlier.
Anyway, Papabear, to your point, Mesopotamia was first populated by Sumerians, who seem related to Elamites and the Harappan civilisation.
Most likely these were all Dravidians -- the Sumerians called themselves Sa-gi-ga or the black headed people
Now, the Dravidians are Caucasians, just not Indo-Europeans (just as Semites are Caucasians but not Indo-Europeans)
So your point about Egyptians being Caucasians (through the AfroAsiatic branch that includes Semites and Berbers), yes, but not to the Indo-European/Aryan branch
actually, Egypt’s southern (”upper”) kingdom developed first because there was a caravan route from Thebes to the Red Sea, and also links to the very rich western Oases and to the Nubian/Ethiopian kingdoms.
The western oases theories are just being excavated, but the theory is that the folks in the savannah didn’t move into the nile valley until the Sahara dried up 12 thousand years ago.