Dominions 3 lore and nation backgrounds !

[LDiCesare]

There are a few things I think you're missing.
Most nations have a reference to a historical nation, or myth. This can often be seen in leader names.
Ermor is obviously Rome. Pythium is Byzantium. C'tis is more Babylonian or Egyptian (animal headed gods, eater of the dead, undead chariots lean towards Egypt, while names lead towards Sumer).
I disagree with Arcoscephale keeping the same units in each era. EA and MA have little in common. MA lacks seducing witches and flying troops. History-wise, LA also introduces ape warriors, so Arco has had contact with Bandar Log or their successors somewhere between MA and LA.

Another big thing you didn't mention is the weakening of Blood magic in Middle Era. Mictlan, Hinnom, both lack blood in MA. Lanka vanishes, leaving Bandar Log as only heir to both them and Kailasa. Pythium (formerly Sauromatia) loses blood too. Man, more or less Marverni's heir, also lacks blood. LA sees a blood comeback (f.e. Mictlan, Marignon)