I think the problem is not about "how big one event can effect a civ" but more like "how some civ got lucky boost while other got the short end of the stick".
I have a suggestion. Is there any way Gazebo can code some certain events (those with big effects mostly) to trigger for all civ at the same time (randomly, or time/turn based) and nerf city-specific events to balance things out ?
Stuffs like "a meteors rain can be observed from all the over the world, some science focus civ take the chance to study astrology (+ big chunk of science) while some religious civ choose to claim it's god's sign (+ big chunk of faith)" or volcano spewing ashes or earthquake/tsunami/eclipse/solar flare/el nino/aurora... that give all civ present the same set of choices, but they're still random so that you can't prepare for them beforehand (thus, still "event").
If those big events happened at the same time for all civ it's less rng whereas some civ would get lucky with a huge boost while other fall behind, it'll all depend on their own choice during the same situation (and what they can actually do, based on tech/building, which would help diverse one game from another). Even if your civ failed to get the best out of an event there're still expectation some other civ might have gotten this and that, and when a religious is found on turn 50 you will know why instead of spewing curse upon rng.
The nerf for city specific events is to reduce the effect of rng, or else with both the world events and city events heavily affect the game you might as well play blind and pray. It'll still be there, but more for random challenges and flavor rather than to make or break a civ. Also if you simply nerf all civ/city events there's no need to take a lot of time "rebalancing" them (since they no longer have big impact to one's game), and people can add a lot more of those flavorful events without worrying about breaking balance.