Expansion

I’ve now almost completed 5 city regions but I’m nowhere near the population requirement to unlock the last region. I’ve spent a long, long time creating aesthetically pleasing cities and don’t want to just turn them all into skyscraper cities just to reach an impossibly high and seemingly arbitrary population demand for the last region. Suggestions? Anyone struggling with the same? I raised this with customer services but they were unable to help and directed me here.