reduction in grow rates only will be not enough i think.
If you reduce only the grow rate so you will have all city´s large or huge after 30 years and not 20 as now...its no really way to fix it.

the only way i think is to allow only few cities to going large or huge at best you allow only lv 3 palace for most cities, for some desert oase´s only lv 2 or 1 and for few like rome,athen,karthago,antiochia etc. lv 5.

Same way as in RTR Naval and City Mod where most Cities can go only to lv 3 (and have no walls too, a other point that were good for EB)