So one of my favourite things about Bionicle is The Plan, Teridax's grand scheme to overthrow the Great Spirit. The audacity of the whole thing is only matched by the spectacular success.
But I don't think the Plan was finalised from the jump. I think 2004 was originally the entirety of the Plan, and he only scaled up to literally becoming Mata Nui later on. Based on conversations I had in Red Star Games' Discord server (and checking The Mutran Chronicles and seeing that they didn't evolve into energy beings, and thus become capable of possessing people until after Teridax's coup and the commencement of the Plan), this is how I think things are laid out.
Thwarting the League of Six Kingdoms' revolution teaches Teridax two things: Mata Nui doesn't defend himself and the rest of the MU don't respect the Makuta at all.
The Matoran Civil War almost kills Mata Nui and Makuta has to clean up the mess again. He now knows that Mata Nui can die and the Matoran working in Metru Nui have something to do with his health.
Mutran discovers from Tren Krom that the world is a guy. The former two points start making a whole lot more sense and he realises that he could actually take out Mata Nui and rule over the universe.
He makes the initial form of the Plan, which is simply to put Mata Nui in a coma with his virus, then brainwash the Matoran to serve the Makuta (him), so they (he) can rule from Metru Nui unopposed, with the Matoran's continued labour keeping Mata Nui alive, but in stasis. Yeah there are Toa that are supposed to revive him, but Toa die easily, he can handle them.
He pitches this version of the Plan to the Makuta, and usurps leadership of the Brotherhood. It is unclear how many of the Makuta besides him and Mutran know about the true nature of the GSR (in fact I think there's solid evidence that Krika and Icarax didn't know, but I haven't gotten to the 2008 books yet so I may be wrong), o what exactly he tells them is unclear.
Teridax puts the Plan into motion, and the Makuta make their preparations. There's no way to subtly crash the Great Spirit Robot, and when they realise what's happening, everyone's going to try and stop them, so they're going to be need to be ready for war.
The Makuta evolve into pure Antidermis. Somewhere between this happening and the Plan kicking into high-gear, Teridax realises that he could possess Mata Nui's body at the moment of his death. He decides to not do that because the risk is too great and the timing is too tight and remains focused on Plan 1.0.
Plan 1.0 fails because of Lhikan and the Toa Metru. His first contingency (the Visorak Horde) is to get Sidorak to keep the Matoran in Metru Nui ready for things to resume while Roodaka works to free him. This also fails because of the Rahaga intervening and helping the Toa Hordika defeat his agents and take the Matoran out of the city, though Roodaka does manage to get him out of the Toa Seal.
His attempt to recover the Vahi fails because Vakama would rather break time than give it up to him.
This is where Teridax stops completely underestimating Toa. Like he still does that a lot, but he has become aware of the one thing they have over him. He can outsmart them, he can overpower them. But he realises that he cannot out-crazy Toa, they're always going to pull off some impossible win. He also knows that the Matoran being out of Metru Nui is going to worsen Mata Nui's condition.
This is where he pivots to Plan 2.0, deciding to weaponise the Toa's crackhead determination (the thing I think Krika and Icarax never understood, hence why vague threats and attempted murder didn't dissuade the Toa Nuva the way that just going "Guys, Teridax's entire plan hinges on you winning, you need to stop him before you wake Mata Nui" would've). He is now actively gunning for the role of Great Spirit, and this is the latest point where he can decide that he's killing the rest of the Brotherhood to protect his own power.
Post-Cataclysm, he has to briefly re-establish the pecking order within the Brotherhood, which results in his fight with Icarax. This is also where I think the most logical place for the "Water Wraith" monologue from Into the Darkness fits, using it as an example of how they're going to win by losing. Officially this speech happens during the reign of the League of Six Kingdoms, before Teridax ever took the the throne, but I'm chalking that up as a timeline flub because the scene clearly frames Teridax as already being the leader.
Teridax shifts his primary focus to stalling the Toa Mata and the Matoran's re-entry into the GSR, allowing Dume and co. the opportunity to repair Metru Nui so that it's ready to go as soon as he needs it, and his armies ample time to rampage across the the GSR and worsen Mata Nui's health in a more controlled way.
Once he knows that the GSR's systems are failing, it's time for him to let them back in. Unfortunately that comes in the form of Takanuva, someone he was hoping to avoid the existence of, but fortunately it comes in a way that allows him to fake his death, affording him the complete freedom to do as he pleases.
He sends the Piraka to get the Ignika because that thing is super-cursed and they're expendable pawns. It's unclear who would end up using it if they succeeded, but it doesn't matter because they fail.
The recovery of the Ignika from the Pit is the most tightly-timed part of the whole Plan, so naturally he decides to do it himself.
At the same time he sends Icarax to make things ready for the Rising on the Brotherhood's end, since Icarax is basically unstoppable, and he can be sure that this stuff will get done.
He also sends his eight best to Karda Nui, simultaneously providing a convincing token resistance for when the Toa Nuva get there, and also setting the eight biggest threats to his power up to be instantly killed when Mata Nui's systems reactivate.
He also orders the Brotherhood to prepare to march on Metru Nui for the final phase of the war, making it look like they're trying to just conquer the universe the old-fashioned way in the last few days before the Rising.
He has no issues with revealing his identity to Matoro because he knows that Matoro is going to die at the end of this. His repeated needling of Matoro and undermining his quality as a hero is likely also in service to this, he's basically preparing Matoro for the sacrifice.
While he threatens Matoro to keep his identity secret outside of the two of them, it doesn't really matter if he tells the other Toa Mahri because he's fully expecting them to die.
With the Toa Mahri racing for the core with the Ignika, he takes it upon himself to stall the Barraki for as long as possible. This is one of Teridax's most fascinating moments in the series, IMO.
The Barraki destroy his body, and he vacates the Pit as the Great Spirit dies, putting his faith in Matoro and the other Toa Mahri to succeed. Again, I love this stuff.
Matoro goes into the Core while the other Toa Mahri decide to sacrifice themselves to let him succeed. Jaller eventually goes Nova, and this is almost certainly the outcome that Teridax wants- either the Barraki kill all the Toa Mahri, or Jaller kills everyone in the Pit except himself and probably boils Aqua Magna's oceans- he's outside the GSR so the full power of his Nova Blast is undampened, and those things can at least render a planet incapable of supporting life when not contained. Either way, the only people/person who knows he's not dead are trapped at the bottom of the ocean and can't tell anyone else (ultimately he reckons without Matoro saving the other Mahri and Brutaka being let out for good behaviour, enabling them to warn the others that Teridax is still out there).
He makes his way to the Core Processor and waits. The Toa Nuva succeeding will be his master-stroke. Not only does it awaken the GSR with him in control, but it will also kill his greatest rivals (TSO not even making the Top Ten list over guys that actively work for Teridax and want him to succeed, oof), and most of the Av-Matoran in the universe, drastically reducing the chances of a second Toa of Light appearing. And it will trap the Toa Nuva in the Codrex, presumably so they can be re-canned in case of a second Cataclysm.
Icarax and Krika's efforts to stop them not only fail, but make things easier for him- Icarax trying to destroy the Codrex convinces Tahu, Gali, and Onua that they don't have time to do things the proper way, they have to get Ignika to sacrifice himself instead, which gives Teridax a convenient place to stash Mata Nui's brain.
The Makuta in the core die. Teridax drowns the Makuta fleet attacking Metru Nui himself. The Order take out most of the others, and the few survivors are harvested for all the Kraata they can produce before being killed. There is nobody left to stop him.
Sure, the Toa Nuva escaped on their new vehicles, but that's hardly going to matter to him now that he's the Great Spirit, right?
