I wonder it has not been mentioned before: Unique buildings should have the highest priority, followed by manufactories. Only when manufactory costs have risen too much, the player should wait a few years and spend the time increasing the country's manpower. Doing so will lower the building costs for manufactories again.
As was mentioned, magistrates are the usual limiting factor in larger empires. Therefore all should be undertaken to increase the magistrate generation, including:
1) the choice of government form (become empire as soon as you have 30 cities),
2) expansion of the sphere of influence,
3) building colleges in all those high-tax grain provinces,
4) charter a Guild of Saint Luke,
5) centralise as much as possible,
6) stay at / regain +3 stability,
7) be at peace as much as possible.