Megacities are easier to make and OP more then ever in this game.
But there is a catch in early game. You need to move only your culture inside for pops to promote and move out others so that your desired ratio of freemans is not filled with other cultures cause this will choke the promotions.
Early game - push your culture pops inside and move newcomers with other cultures to fill the food area with slave promotions off .
Buildings , farms and slave estates in settlements .
Aquaducts only in targeted city cause what you need at start is capacity and numbers for your pops to promote faster, more pops faster promotion but be aware of choking it with wrong cultures beeing promoted to freemans .
Ignore assimilations and use social mobility to increase the promote/demote speed.
late early game 467-490, you are out of your culture pops, at least you should be if you dont play macedonian culture nation, everyone is in that city.
Build up in slow fashion , 2 aquaducts 1 theater . Move some assimilated slaves in to increase the pop base. More pops faster promotion is, more nobles you have ,more trade routs you will have.
in year 600 i had like 150 trade routs , have used maybe 100 of them, not all on food, maybe 50 was food. What you need is a temple and metropolis, temple will give you 3 artifact slots , there is 3 of them which are giving 10% local food modifier so place them in that temple and each food good will give you 30% more.
At that point influx of pops will be huge if you are offensive player + migration attraction , advisable is to drop some foundry , like 10 of them is okish.
Worthy to mention, diefy your rulers asap, use the conversion idea. If you have 4 of them plus idea + there is a tech for 10% and you can use law for 30% all your pops will arive as your religion guys in the city sou you dont need a single temple to fonvert them. Speed of conversion can be up to 7 % in not a single pop of your religion in teritory , you can speed it up with policy which will be preferable by governors on default do the conversion bonuses what you have.
So late game would be like 5 aqua, 1 theater, 0.5 foundry