So I have figured out what mainly drives the Rome 2 DeI economy would be the screens below. In particular the empire maintenance and the tax modifier of the province itself battle it out and the resultant percentage is what directly impacts the income from each province.
So a province has a certain number of regions generating wealth from different types of income generators; subsistence, commerce, maritime commerce and industry. The total of this income is then multiplied by the difference between the Taxes modifier and the Empire Maintenance modifier. The formula is as follows;
Province Income = (Sum of Region Income) X (Taxes - Empire Maintenance)
So in the above case = (2,647+2,750+3,320+3,458) x ((100%+34%)-96.5%) = (12,175) x (37.5%) = 4,562
The part of the formula in red has the highest impact on the Province Income generated, and so improving that value can change the outcome drastically.
In the example below in the same campaign I own only two provinces in Africa.
Province Income = (2,393+2,862) x ((100%+86%)-96.5%) = (5,255) x (89.5%) = 4,702
So, despite owning fewer regions in Africa, I make more Province Income than I am making in Latium. In another example below of Asia Minor province.
Province Income = (2,637+3,419+3,675+1,150) x ((100%+75%)-96.5%) = (10,881) x (78.5%) = 8,538
As can be seen, the Regional Income of both Africa (5,255) and Asia (10,881) is lower than Latium (12,175), but due to the difference in the modifiers (89.5%), (79.5%) and (37.5%) respectively, I am making much more in the former two than in Latium.
While empire maintenance is a flat percentage based on various factors, it is constant and applicable throughout the empire, and can't be changed individually from province to province. What can be changed is the Taxes modifier and for that purpose it is the most important economic modifier in the game. Now that I established that the question of this thread is the following:
"What affects the Taxes modifier?"
If I know the exact answer to that question I will have a better shot and maximizing my province output. Thanks to anyone who can help me solve this. I know increase the tax rate increases that Taxes modifier by an increment of +20%. Which means if I am to assume Lowest Tax increases it by 0%, Low Tax would be +20%, Medium Tax would be +40%, High Tax would be +60% and Highest Tax would be +80%. While that would be the easiest way to improve the Taxes to Empire Maintenance ratio it does come with its drawbacks to public order, population migration and settlement growth rates. So, I am really interested to know the other hidden factors in the game that affect this Taxes modifier, in particular the more variable ones, not the flat bonuses from technology, etc.