need a lot of help with my world's history

I’m having trouble with my planet’s history. I have over 9000 immortal kings and queens in history and they’ve all had one rule to follow. The rule is every ruler first in line cannot be eligible take the throne until they are over 1018 and their predecessor has stepped down and every ruler who has ruled must step down exactly 100 years earlier than their predecessor.
For example:
King Raphel ruled for 999,000 years
Queen Amora ruled for 998,900 years
Queen Lily ruled for 998,800 years
King Sanders ruled for 998,700 years and so on down the line

I am currently writing out all their ruling years and adding them up so I can figure out exactly how old my planet would be because a new era starts when a new ruler takes the throne.

I did it this way because I wanted my planet to be old as hell, and for my main character to have an old as family heritage but I’m starting to doubt my decision because it stopped making sense to me. They’re all immortal like gods I guess and the people that live on the planet are like the equivalent to humans they watch over. But one person doing it for hundreds of thousands of years seems really boring, and I’m more or less concerned if it would be realistic if it stayed that way for so long and nothing about that rule changed.

I’m also worried it won’t make sense, like why was this rule made in the first place, even I don’t know but the only idea I have is that the first ruler wanted something like a timer? For something?

My story starts when the ruling years get down to around 2000 and that’s when they finally break that tradition so it’s been that way for a LONG time. And I don’t know if I should keep doing what I’m doing or scrap it all and start over. But I really don’t want to do that because it took me almost 2 years to write everything down like this.

Am I just overthinking this whole thing?

While the length of rule is certainly unique, it's incredibly rare that anyone could rule for so long. People can change a lot over just one century so for one ruler to rule for nearly 1 million years is unrealistic.

I don't know how long your human equivalents live but, assuming that they live about the same length as humans, I think the main problem you would encounter here is how much people change. Culture, religion, society, civilisation, belief. It would all change far too many times during just one ruler's reign. A thousand times over or even ten thousand times. If the entire world's population was immortal or at least incredibly long lived it would be believable.

I think what's important is to not underestimate how long, say, one thousand years is. Ancient history was only a few thousand years ago. The first civilisation began just over 4000 years ago.
A problem I faced in worldbuilding was having too much time pass. Like a said, a lot can happen in about one or two centuries. Empires have risen and fallen in that timeframe. In a fantasy world it's fine to stretch this, of course, but just keep in mind that shit is always going down.

I wouldn't say that you'd need to rewrite everything. Perhaps you could just increase the amount of rulers and shorten their rule to something a little more manageable like 10,000 years. Even in that way the ruler can change with the times, at least in a fantasy setting. In the real world this would never happen…never mind the immortality part. There's nothing wrong with having a long history and an old world but people are people so it's something that should be kept in mind.

I hope this helps a little

@Reblod thank you for your reply, it really helped sort out most of my worldbuilding problems.
I just have one question to ask if you want to answer, if you don’t, that’s totally fine.
My human equivalents natural lifespan is about 300 years, how much do you think this would change about the world in general?

Well, from how I understand it, the longer lived something/someone is, the more patient they would be. But it depends on the society I suppose.
A longer lived society of war-faring or generally tumultuous people would be just as turbulent as a human's much shorter lifespan. A more secure, peaceful society would change much less. In general, I think that there would be more time to spare for anything - the development of culture, aggressive or peaceful action, arts, etc.
I assume you mean how the world would change in comparison to our world? I would say that civilisation would be a lot older. I do, however, think that perhaps they would be slower to advance technologically and culturally considering that there would be less new minds to help innovate. It might take a long time for old ways to be shed unlike here where every decade has a new culture.

That's my perspective anyway