The one thing that made the greatest impact on my economy are farms. At first I was reluctant to build them as in RTW squalor could easily spiral out of control and make cities a burden, but in M2TW farms are nothing but good.

In M2TW squalor can NOT go over 80%. It's capped.

Let me repeat: there is a squalor cap! Which means that once you reach it, farms and increasing population is nothing but good for you as 80% unrest is easy to overcome with some churches/inns/etc. Over time, farms end up being a huge source of income, easily an additional 25%-30% of your total income if you develop them. That makes a big difference.

Build farms first, then roads/markets/ports. If you focus on the economy and keep your city:castle ration at 4:1 or better, you will be swimming in cash. You literally won't be able to spend it all.