A question about Seleucid economy

[king of thracia]

I started a new campaign on H/M, one of straightforward conquest and I'm swimming in money. I keep the minimum military in the field, but not so minimum that they can't win battles. Once I'm past the start I set all taxes to low and develop economically except for fundamental military infrastructure at key points. Lots of low level trade docks, farms, basic granary, and then estates and highways. I build the first level of mines after basic sanitation, farms, and trade docks are constructed. Strong coastal trading areas use Type IV governments until they have basic development; exception is of course Antioch which in addition to being already highly developed is the main military base of the west. And Alexandria, now that I've conquered it :). The eastern frontier is switched all to type 2 governments.

Now, keep in mind that despite its vast territory, the number of cities in Arche Seleukeia is not especially high. A Rome that is expanding out of Italy and a Getai that controls northern Greece and western mikra Asia can already rival in number of cities. And Greece and western mikra Asia are especially lucrative.

For peace of mind you can wall off the northeast with Baktria. Attack Pahlava first to keep Baktria as allies and make them break with Pahlava. It's possible to have relatively peaceful borders in the west without expanding much beyond initial starting position; I recommend taking coastal regions at least for economy. However straight up conquest of Ptolemies, mikra Asia etc works out well as you may know.