This may not help much, but I consider "a good income" to be any situation where I don't even think of how much it is, since I always have enough. At that point the concern is simply to stay below 50000 in the bank so as to avoid negative traits on my governors (bribing small rebel stacks works well).

Re: artaxerxes situation, all I'd add is to prioritize the mines in Karmana and Gabai over everything else. Those two mines are the economic heart of Persia, since each province contains two mine sites (so double the income of a normal mine). A quick strike at the province we would today call Eastern Saudi Arabia could also be worthwhile, since that will more than double the sea trade in the Persian Gulf.

And don't disband all your cavalry, just the ones you don't need. Anything weaker than Prodromoi should go, but keep Kinsmen or Hetairoi (unless you've managed to spam a lot of them). Top tier heavy cav are expensive, but well worth it. Their heavy armor will usually keep their casualties extremely low if used well. And of course keep any Parthian horse archers - both cheap and deadly.

Hmm, thinking back to my old 1.0 AS campaign, I remember what jump-started my economy was sacking the unique temple in Ecbatana. I think I read that it actually does something in 1.1, but back then it was a piece of decoration that I could tear down for 17000 mnai. That helped. You can do the same with the Cedars of Lebanon in Sidon - 10000 mnai.