Thanks for this info. I had decided before seeing this guide that I'd stay at level 10 just because it should be easier to get keys and expansion items, even though I will probably be at this level for a few months. What I didn't realize until reading this guide was that tax per capita decreases as population increases. Now i know to not buy another bus terminal and just save keys for the heliport.
Thanks for the excellent tips.
Also, although it's annoying staying at 10, it's something I got used to. I can still advance my city by buying upgrades and I'm still making progress by slowly opening inventory and zone expansions, so it's not TOO bad sitting at 10 once you get used to it and realize it will make future levels less of a hassle.
Cheers!