The thing that helped me was to think backwards. Start where the story starts. It’s possibly quite solid so start where it’s solid. In other words, give yourself a break Start where you know your stuff.
Since your world changes 5000 years at a time, you might want to first list up major events in order of occurrence starting with the most recent big event that happens in the story. The point of the timeline when you are making it in the beginning is to kind of outline the timeline. Like you said, only think about major events that affect the plot.
A war that separated countries, a peace treaty, if you have a kingdom maybe a king takes a throne or falls from it, crash of economy could be a big event, an earthquake, a leader goes on rampage, a flood drowns thousands…
A fictional history, if you want it to, can have a bunch of natural disasters (I mean, look at the history of our world). You have the weather at your fingertips and earthquakes can happen if you make them. Feel free to administer natural disasters or plagues in your timeline No one is stopping you.
Keep in mind that whatever events happen logically lead to other events and it has to make sense in your world. So, if you have the fall of a kingdom, someone has to come up and take over the land. If you have a big famine, the economy might be affected. If someone changed the law, that might increase or decrease criminals. Cause and effect Even a fictional world has to logically make sense history-wise. BUT only events that have to do with the plot. If the crash of the economy doesn’t have anything to do with the plot, be nice to your brain and don’t think about it XD
After you’ve gone back in time as much as you need for the story, you can go back in the timeline and add in country-by-country details if they are important for your story. If it’s not important for the story, don’t overwork yourself with the timeline You don’t have to mention the characters at all unless their actions affect the big plot (like, don’t mention Betty changed her hairstyle, for example, or that her parents got divorced). It’s already a big endeavor to make up an entirely fictional timeline. I scoured through our world history timelines to figure out how it’s supposed to look.
Here’s an example for working backwards:
The bold is the main events. I suggest start with this kind of list. If you need years and dates, you can always go back and assign those later
Timeline example thing
Main story event 3 (most recent, possibly the thing characters are dealing with for the majority of the story)
main story event 2 (caused 3)
main story event 1 (caused 2, least recent)
something that caused main story event 1
Citizens of Country B revolt
Country A and Country B are at each other’s throats and A takes over B.
Big World War caused by Country A
something that made Country A mad
another country does something
An unnaturally hot summer
Something like that. Ask yourself, “what could have caused this?” when you think backwards.
This is based on my experience with making the first timeline I mentioned which I did quite recently. If you need to talk more, I’m free
Btw, this isn’t the words of an expert, but this way of making a timeline did work for me. It was after two failed attempts that I figured this out. Try it