The main problem here is level 12 and the unlocking of the Police Service. That, coupled with the pressures on your water, sewage, power and road congestion levels, will leave you seriously strapped for SImoleans from level 12 onwards.
The fact is you may not be able to provide total police coverage for all your inhabitants straight away, so the happiness of your City will drop.
The only suggestion I can give is to work through the building requirements and earn enough SImoleans to pay for complete City coverage by your police services (and fire services) this may mean up to 3 basic fire and police stations!
Thankfully Health is not unlocked until level 16 so you have some opportunity to raise enough Simoleans to pay for all of these services and make you Sims happy again.
Try not to keep your City lacking both Fire and Police Services, I think it is better to have complete coverage of one or the other, than have uncovered bits of both.
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Plan your residential zones so you are able to get 100% coverage of police and fire services. Maybe this will mean not utilising all of your available residential zones.
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Although the commerical units cost a lot, especially during a time where you have to make lots of provisions for police and fire serices (and from level 16, health). The commercial units allow you to craft more items which can be sold on the market, and also to fulfil more varied building requests.