Re: forts and income bonus
I haven't done the math, but it's not a no-brainer to get forts everywhere because you get an income bonus. Forts aren't free. In fact, if you prefer LA as much as I do you're generally paying 800-1200g for one. It takes a long while for that investment to pay itself back if I'm concerned mainly with income. You could probably crunch a few numbers to calculate some ideal amount of turns after which a fort has effectively paid for itself. But before that special turn, you're losing money by building forts (of course you get awesome things for forts so you build them anyway, just not for income reasons).
Another question, what's the best way to maximize income? As far as I know it's order3/production3/growth3, no temp (or perfect for your nation), and no unrest. Anything else major that I'm missing?
Assuming similar amount of provinces, that's indeed what keeps wealth rolling in.
But it's never the same amount of provinces. The guy with an awake dragon (or similar) will get one province per turn without any attempt at expanding with anything else. That's easily ten provinces by the end of year 1. And you can expect another 5-15 from your national troops with good expansion pace. Other option is to get a good bless (simple X9 is usually good enough) and expand like crazy with your sacreds. You don't need either though, some nations are rather good at expanding with just scales as long as they have enough income and production (favourite of mine being EA/MA Pangaea and its centaur warriors). Conventional wisdom is that your expansion is going to be slower if you don't have an awake fighting god, good sacreds or good national expanders.
Also be mindful of your upkeep. Everything you recruit should be for a purpose, and ideally cost effective at that purpose. Troops that just sit there aren't earning their keep (well they might act as a deterrent, but that's difficult to appraise). Troops that are constantly marching and fighting and winning and dying don't weigh on your treasury as much as idle troops. Of course you can't usually create a full fighting strength army in a single turn, so what I wrote above is more of an ideal one should strive towards rather than hard reality.
In an infinite game, mages with low upkeep/RP are preferrable over high upkeep/RP mages when it comes to research. This is generally opposite for battle mages, thought it varies. Of course no game of Dominions is infinite, but generally speaking, if a unit is going to sit in a province for fifty turns I'd prefer that unit to be cheap.
And tying into that, don't over recruit. I sometimes get a bit trigger happy with forts and as a result I end up recruiting far more than I can comfortably afford. It's happened once or twice that I've had a billion useless highly efficient research monkies who drain my income every turn and thus prevent me from getting stuff done as much as I'd like.