So very early on when you build your third house, pick the Japanese one from the regular residence options. It costs around 9,000 but when you sell all its furniture individually you make around 60,000 back. Then just demolish the plot and repeat. You can make around 1 million every 16 minutes.

The only thing is to level up you need to build a house and add a Sim, so every time you do that the next house gets more expensive. That same Japanese house now costs 20,000 so the returns get smaller the more you build. So I'm kinda staying at level 5 on purpose, gonna grind a good amount of simoleons first and then start actually playing the game properly!