Here are the keys to a good economy:

watch how expensive your standing army is. If you own lots of cities, you can usually keep some town milita (or better) troops there for free. Always do so. Free troops are nothing to sneeze at, especially when your empire is FULL of them. Any non-free troops sitting in your cities are usually a waste of money unless they are cheap and you NEED them to maintain order or defense. I usually don't waste time defending anything unless it's really important. I can always reconquer little provinces later.

Make sure you are being cost effecient with your invading/seiging armies. Mercenaries are great for crusades, but only for emergencies otherwise. They are almost always 1.5 times as expensive to maintain as any other troop. On the empire scale, thats a LOT of wasted money.

Make sure you have unblocked ports and roads. I found that sometimes a market doesn't do anything. Glitch?

Castles don't earn squat. Make lots of cities unless you're feeling wimpy or overly aggressive.

look out for invaders and rebels, and corruption. Any army standing in your territory affects your trade.

If your conquests take too much money to maintain and properly staff, let it revolt and then exterminate them again and convert the populace with priests/churches. Less garrison troops means less expenses.

Look at your budget scroll. What's the most expensive stuff? Eliminate it.

You automatically make money. You go bankrupt by spending too much and not upgrading your trade and cities.

I have more suggestions... please message me if youre interested.

AND PS:

Merchants are fine, but only use them when your empire is massive and has expanded far from your capital.

Then what you do is pop out like ten of them and suck up the resources around your furthest big city. Silk, spices, etc very good.

Byzantium is a great place to pop out merchs because of the 4 silks. Antioch is near cotton, sugar, and spices. Russia is surrounded by amber, which is useless unless it's far away.

And buy off little merchants that you see. Save first, and attack. Make sure it's even possible, check both ratings first. You get a bonus for buying a merchant off because you seize their biz and get their money.

Merchants can earn you lots, if they are good, but never as much as sacking cities and being cost effective.