After rewatching, I felt the need to exhaust my thoughts on this anime, and this seems to be the best place to do it.

I always thought Code Geass was not as good as it could be. I shared the opinion of many fans about how only the ending is perfect and the rest is just good. I had thought this even though I watched the whole series in under 2 days and I have seen other people say the same thing. But how could everyone do the same mistake? Yes, because a geass was cast on us. Rewatching Code Geass 14 years after I initially watched it, it made me realize how good of a show it is.

Narrative Structure:-
You see, the events happening in Code Geass always outperform the events happening in the previous episodes in terms of scale and grandiose in an obtrusive way. The new event overrides our memories of previous events, and the past episodes seem insignificant in comparison. Isn't it ironic, this is how Geass works. The ending of the anime was so strong, and the best and most complete ending for any anime ever, everyone thinks only the ending was good and other episodes were boring. Think about it, haven't you felt that you like a character but they do something so stupid you start to hate them the next episode, or the opposite.

Characters:-
Zero: A public masked identity, originally created and assumed by the former and exiled prince, Lelouch vi Britannia.
Zero can be pronounced as 零(Rei) in Japanese. Make no mistake, just because they use the borrowed zero doesn't necessarily mean they weren't thinking about wordplay. Writers love their wordplay, and the Japanese have quite a strong tradition of it. I'm not really qualified to talk too much about other languages, so I won't do so at length, but you might want to poke around something like wiktionary to check out some of the extremely interesting potential readings you can get out of the れい sound depending on reading and origin without even leaving Japanese. (The king bit is certainly true, though - Spanish rey, Latin rex, French roi, Portugese rei..)
From English, zero has a lot of potential metaphor to it - absence, emptiness, void. But lest we forget, its etymological root is in the Arabic sifr - from which we also get cipher and its metaphor of secrets, disguises, and codes, as well as the intriguingly adjacent royal cypher .

Lelouch: The story is structured around him and the anime excellently makes him relatable. He is a charismatic, convincing and inspiring, as if casting a Geass to the viewer through the screen. I am not really sad about his death, but I can only hope that the people close to him understands his actions and remember him well.

CC: She does not contribute much to the story. But I think we can all agree that she is a really charming character.

Suzaku: He is self-righteous and blames everyone but himself. Although Lelouch often accepts his own errors, Suzaku comes up with an excuse for his actions. Even the reason Japan's terrorism is caused by Suzaku killing his own father, but he fails to realize it. At the end, he finally accepts that the way he did things was wrong and joins Lelouch.

Schneizel: Charismatic and manipulative, cunning, overconfident and ruthless, Schneizel is widely regarded as Lelouch's greatest adversary, including Lelouch himself, and he serves as one of the primary antagonists of the series.

Charles: He is a cold and intelligent man who strongly holds views of social Darwinism, believing that only the strongest were worthy to rise in society his views and ideology seem to have stemmed from his traumatic upbringing and serve as one of the primary antagonists of the series.

Nina and Nunnaly: I think they deserve the hate they get. As Lelouch said, "Those who shoot should be prepared to be shot". And I think Nina and Nunnaly got away with so many head counts even though they weren't prepared to get shot. That is why the best thing we can do is hate them.

The power of Geass:-
Code Geass took lot of inspiration from mythologies.
The geas , literally meaning "taboo" in the Irish language, is a reoccurring element in Celtic mythologies. It refers to a strong curse or magical stipulation placed upon an individual which either compels them to act in a certain way or forbids a particular action. Though obeying one's geas can result in strength and blessings for the one sealed with it, breaking it can lead to severe consequences -- up to and including death, as was the case for the folk hero Cú Chulainn.

In the anime a Geass is created when a code user makes a contract with a human. The Geass manifests differently in each individual, possibly related to their inner desires and personality. The power of Geass increases with use, and should the user lack the willpower, they may be consumed by it.

Plot and the Ending:-
The story of Code Geass begins in the totalitarian run alternate history. In 2010, the Britannian Empire enslaved Japan using powerful mecha known as Knightmares; in the aftermath Japan was renamed Area 11, and its people began a hard and terrible existence. By Using the supernatural power, known as Geass, our protagonist leads a rebellion against the rule of the Holy Britannian Empire to free Japan and to avenge his mother's death, commanding a series of mecha battles.

zero requiem is the best solution, the anime has from the start developed to converge to this solution: the problem is not only about britania or the monarchy, war has been since humains exist C.C said in the last episode, people tend always to fight for what they think is right, but they never try to understand the other's pain nore they care about them, when charles died, Schneizel came, and this would have continued endlessly, but the reason why the zero requiem is the best solution is that he centred all humanity's hatred toward one person, they were finnally able to understand the other's pain cause they are all under the same treat, lelouch took over the whole world to unify people against him and to make them understand how peace is precious and that fighting is not the solution, so when lelouch died, they were freed from the burden of hatred and they could sit in round tables to discuss their problems. Code geass doesn't say too much about these things, it only hint to them so that the viewer could make his own vision of the thing, and i find this to be brilliant. They hinted the ending from ep 5, lelouch said back then "when does war end? When some one wins" he also said in the first episode " the ones who should kill are those prepared to be killed" and he also said that he will destroy the world and make a new one and defeating evil by the means of greater evil.this is lelouch's philosophy, he doesnt believe in mere words, like "i will change the system from inside" and the anime has shown that it's impossible this way cause the whole system is corrupted and susaku finnaly understood this and joined with lelouch. There is also the thing you said about them wanting to atone for their sins, it's absolutely true but it's only one part of the whole thing.

Now about what Charles and Schneizel were up to, charles is some one who is living in the past, he doesnt care about the present, he dedicated his all life to create a system that could unify all people's mind, the incarnation of this is the thought elevator, it's only a symbol you know, but for him to achieve this goal he needed two codes, he already have V.V's one and from the begining of the show he was after c.c who didnt agree with this plan, you remember the first episode when Clovis captured C.C and was about to give it to the prince, but the japanese stole the capsule from him. The sword of akasha is a symbolism in the japanese folklore, i dont have a clear idea about the myth but in code geass it incarnate how god will be slayed, god here is the collective consciousness that were gathered via the thought elevator. The purpose of all this is to to get rid of the lies in the world, by connecting every one's thoughts, so that every one will act as he is, without any mask. But the problem with this system is that it dosnt allow people to evolve, cause they will be no desires, no problems no future, life will loose its value cause even the dead people will be in that world. This is why lelouch rejected this world, he wanted a dynamic world where people long for the the loved ones, where they hope for the better, where they struggle to achieve their goals, even if they lie or commit mistakes, humanity should not stagnate. And to defeat Charles, mariane and their system it was so easy to do and Lelouch figured it so quicly, yes he couldn't use geass on charles who has the code, but he could use it on the collective consciousness as it's made from people's minds,( remember geass work on the mind of people not the corpse or the object no matter the type of geass)

Schneizel in the other hand is the one who thinks only of the present, he doesnt care how the future will be, and this is why his methods are too extreme cause the consequences doesn't mean any thing to him. Lelouch realises the true nature of his brother and was then able to defeat him, he realised that Schneizel doesnt have any desire as he doesnt care for the future, he doesn't enter a battle to win but only not to loose, and because he misunderstood Lelouch's ideology he thought that lelouch was willing to kill him, and that's what brought his loss, cause lelouch from the start was planning to use him for the sake of the Zero Requiem. "Charles values the past, you value the present, but i value the future, cause no matter how the future will be bright as people are longing for the best" said lelouch to Schneizel in ep 24. And to link this the Zero Requiem, lelouch chose to die for the future to be bright.

The high school setting is also used well to appeal to a certain demography, establish the power scale, and add extra drama and enjoyment.

Good & Evil = Zero(Lelouch)
Perfectly balanced.

Overall:
Code Geass is an over the top mecha story with showy twists. It's an classy Anime with a great protagonist and a satisfying conclusion.