If I remember my history well Egypt was basically like China: a more or less coherent empire that was however many times conquered and reconquered or had intern

If I remember my history well Egypt was basically like China: a more or less coherent empire that was however many times conquered and reconquered or had internal political takeovers. So you may be right that 342 is a better date but some could probably argue that an even earlier one should be taken, e.g. when the various mesopotamian empires conquered it. On the other hand the Roman takeover seems to be when Egypt truly became a vassal and lost its independence.