Unlocking a new region only after 10M people seems unrealistic.

Hello Mayor, you have a lot of work to do to get to 10 million. It is possible, I was almost there but then came services 2.0 and my population plummeted, almost back to where it was.

Try replacing low boost structures with higher ones, Sakurai parks instead of small fountain parks, heliports instead of bus terminals and so on. Add lots of high boost mayor's pass buildings. Washington monuments are also good.

Works for me. Good luck.