Most income comes from battle loot, even for AI lords, this has been noticed by TW when one of the developers started investigating why lords were going bankrupt, that is why they decided to increase income from fiefs by a percentage, which I think was 25%.
As for you current income, the first thing you might wanna check are your workshops, what are they producing? As far as I've gathered, checking which workshop is going to be the most profitable is easy, you have to check which raw materials the town has and you also have to check the price for the end product. It doesn't matter if grain is really cheap if the beer it produces is also sold for a very low price.
I would also avoid buying all three workshops in the same town as that town won't have that many profitable combinations and you are going to have at least one workshop not earning as much as it could if it was somewhere else. Having your own kingdom makes this harder as you will lose your workshop if the owner of the town declares war on you. If you are in Sturgia, buying something in Aserai lands might help mitigate this risk.
Before starting your kingdom, try making a large amount of denars, something like 500k, so you can endure negative income long enough to conquer more locations and taxes start to balance your expenditures. Think of it like starting a business, you are not going to be profitable from the beginning so you must have enough capital to pay for your expenses until you can start being profitable.
IMO caravans work best while you are neutral, their main threat will be bandits and they can stay alive longer. Mercenary work also pays well if you are willing to join the losing side, you can hunt newly spawned lords for prisoners and loot and then switch sides once yours gets the upper hand.