in descr_settlement_mechanics.xml you can also tweak growth iirc
otherwise ask the guys in scripting workshop to maybe make a script boosting population only at certain points in the campaign
EDIT: actually now that i think about events there's a very easy solution, the AI bonuses are given through export_descr_buildings.txt so all you gota do is go there and make it require certain events in order to grow
for example in some of the first lines you can read when you open EDB is
Code:
population_growth_bonus bonus 2 requires event_counter is_the_ai 1 and not event_counter thinks_of_his_people 1
change it to this
Code:
population_growth_bonus bonus 2 requires event_counter is_the_ai 1 and not event_counter thinks_of_his_people 1 and event_counter HEAVY_MAIL_ARMOR 1
now the AI will get that growth bonus only when heavy mail event happens which is 1200 AD more or less, you can see when events happen in descr_events althrough you should prob know if you changed turns per year