How to keep population happy?

From this Steam thread:

• Level up districts (city expansion tile) and cities instead of making lines of unleveled districts.
  To level a district or a city, surround it with 4 other districts
• Research +approval technologies and buildings
• Use luxury goods that improve approval
• Use influence for approval kingdom bonus

In addition to that, some anomalies like the spiral rock peel will provide approval bonuses.

You are expanding too fast. The -60 Approval lists "From Expansion Disaprroval", which increases with each city.

You'll have to stop generating new cities and invest in Aprroval bonuses if you want your cities to be productive (since Rebellion results in a huge drop in production of all things).