Maybe changing GoN civic is enough. Non-elves usually don't use it anyway, unless following FoL, and it's bonus to happines and health makes giga-cities possible. Changing it to require from 3 food per population point instead of 4, while providing bonus XP or things like more XP or great general points inside borders or lower support cost should limit population of elven cities in mid and late game, without hurting they early game. And it fits lorewise, after all being guardian of nature often means having to fight with those who want to destroy it and often in fantasy worlds elves have to fight to defend their forests.