Do not fund the schools until 4th Age. Once 4th Age began and you have hit 16 to 18k monthly cash, unlock the funding for schools.

One School or Library can sustain 10,000 population, and can be stacked just like hospitals(since it has no coverage).

To add more information, Since lower Edu level = Less population, try increasing the education level to at least 12 at 400,000 population. This will prevent the city from stagnating once it hit 530k.

for me, this is not an issue. Taxes are the real issue. Try reducing the tax(as long as you get income) from 8-8-8 to 7-3-3 in 220k, 2-2-2 in 350k, 1-1-1 in 450k, and 0-1-1 in 550k population.