I address one aspect of this here. The short answer is that the idea of Achaemenid equitableness is a product of Persian propaganda more than reality. As Amelíe Kuhrt summarizes Darius' self-descriptions:

Darius describes his realm as consisting of a large number of obedient subjects, each of which he rules with perfect justice.

Darius' declaration is rhetorically effective and sticks in your mind I think partly because the Zoroastrian moralism pervading it is very similar to our conception of "ancient fairness and justice" and it coincides with Christian morality on a number of point. The bizarre story of his own ascension is interesting in its own right; I plan on doing a writeup on that next week.

Another aspect that should not be overlooked is the Cyrus Cylinder and modern propaganda surrounding it, which I address here.

I'm currently travelling and won't have access to my books for 2-3 days so it might be some time before I reply, but feel free to ask any followup questions.