HI, I'm currently @ level 17 with 120k residents. And the daily taxes are over 10k. And I easily can make 30-40k in a day just by trading. In the first run of this game I just simp[ly ruined my city with expanding, which cost extremely much. So I had to start from the beginning with a different strategy. This game is actually a test for human patience. So take it that way, this game is excellent to learn how to manage your cash flow, even in real life.
The point is. Build less residential buildings, a lot of factories and some commercial buildings to produce some goodies that you can sell. Go make some calculations about what materials are the best to produce in less time with a highest possible ROI (return on investment). In materials, actually, you are investing time. in some cases a lot of time.so there is a relative ROI. Make some calculations how much hours can you spend daily in the game and adjust the materials for that. when you are in-game do materials that are produced fast but can earn as much as possible selling them at the Trade Depot. Between this materials always produce wood and metal for chairs hammer, nails etc. and if you want some seeds for the vegetables. This will make you more extra coins. While you are in the game tap on the citizen offers. And why not search for goodies at the Trade HQ at the lowest possible price and sell them at the maximum price with free advertising which is available every 60 seconds. PLus you'll get a lot of free stuff while tapping on visiting other cities bubbles. Those things can be sold too. Cargo shipments, and a lot of things.
Before you exit the game, manage to fill your factory with materials that are taking to long. so While you are off the game at sleep/school/work/etc you'll make a ton of materials that worth more coins.
Use your simcash for upgrading commercial building slots. more you gave more you can put to produce while you are not in the game. After you are in the game again you can have a lot of products to sell at a very high price. and easily can make 10-20k just from one shot after a sleep or a work day.
So you can make easily 40-60k in a day. or even more.
The main idea is to not expand fast. There is no competition for the best mayor in the world. So why the rush? Be aware of the instant gratification syndrome. It will force you to think that if you make it faster you'll be rewarded. There is no reward for fast movement. But there is a great educational reward in management. This game is all about of developing the best strategy to manage your city in a way that your citizens are always 100% happy and live a luxurious life. So they can pay you more taxes. Keep the residential zone numbers as low as you can, so all the buildings are upgraded to the highest skyscraper possible. Oh, one another thing. Upgrade all the road to the best available road you have. This will allow you better and more luxurious buildings.
Don't Rush. And you can make a huge amount of coins. And unlock as much education as possible. Beach, transportation. I managed in a way to keep my sims at the highest possible luxurious level without transportation. This feature will be bout tonight. But my sims didn't ask for it. cuz they are at luxurious level, so they have their own cars I guess. I recently unlocked the airport.
So it is possible with low sims to earn pretty much. It depends on the strategy you develop. The fun part is that you can play this game mouths. Be smart, use a little math and everything will be much easier and funnier. And the most important, don't rush.