I moved my capital to Seleukeia, which fits with the story, as Sarpedon does the management from there while Antiochos and Theodoros are on warfare. I have normal taxes whereever i have a unselfish governor and as high as possible elsewhere. The money i earn is enough to maintain a decent army for the Basileus and a smaller, worse one for the heir and to build some buildings, so it's okay right now.
I'm not going to be as systematical as you've been, as it all depends on the preferences of the ruling persons and i'm not going to win the campaign so early :) Perhaps i won't be able to hold together the empire, ah, so be it. The Seleucids haven't done it, neither.

By the way, thanks for contribution, both of you!