Instead of having the events trigger upon first colonization, why not have the events trigger on specific tiles?

Say there is an event on tile 2,2, if the colony is built on tile 2,2, then trigger the event. Or if the player develops tile 2,2, trigger the event.

You can also play around with the tile descriptions a bit by giving hints that there might be something about tile 2,2.

With this system, you can also implement proximity triggers, etc. Something that makes both more sense than trigger upon colonization, and more interesting.

Heck, you can even add infrastructure triggers. Say a nuclear power plant is built next to the embassy, could trigger potential events such as sabotage resulting in death of ambassador(and the surrounding infrastructures), and thus war, etc.

Makes the events system a bit less random, and yet more interesting

Once this system is in place, modders could make even more complex systems such as adjacent tile bonuses for different infrastructure, the possibilities would be endless.