If i'm not mistaken, the Hyksos invaded Kamit during the 15th Dynasty. Pheros was the son of Sesostris III who ruled sometime during the 12th Dynasty. So, it's possibile that it could've been coined after the invasion of those people but most scholars and historians point towards the greeks for the origin of the term. During my years of research, I conclude the same thing. However, I did read the same thing as far as a possible origin of 'Pharoah' from 'Pheros'. But at this time, I cannot confirm that it is accurate. But also keep in mind that the name 'Pheros' is also greek.
Perhaps someone else can shed greater light on this than myself.
Htp.u