The end goal of the Greater Will is almost certainly the Erdtree harvesting the souls of the dead for them to later come and harvest the tree itself when the crop is complete. It seems pretty obvious, although never directly confirmed, as the inspiration is easily seen in old legends. The arena in which you fight the Elden Beast is a starry void filled with hundreds of trees in the background, implying that there are other Erdtrees scattered throughout the cosmos on other planets. Other sources of power for the GW to harvest. The GW is referred to as an outer god, which are above even the great old ones in Lovecraft's mythos. What is confirmed however is that Marika grew to distrust the GW and the Two Fingers, who are essentially the corporeal vassals of the GW in the same way that the Elden Beast is the incorporeal manifestation of it, until you defeat Radagon/Marika and it arises to try and stop you from altering the course of events in the lands between to one that the GW doesn't approve of.
This gives reason for the GW sending a meteorite carrying Astel down to destroy the Eternal Cities. They figured out the truth about the GW and the Erdtree, so they tried to create their own lord to overthrow it, and probably got pretty close judging by the mimics. The two fingers declared it "treason" and so they could justify destroying the civilization since the two fingers/GW is manipulating everyone into thinking they're benevolent by providing a sense of order to the lands between, and great power bestowed by the elden ring itself.
All the other little details add to this. Marika impressing a need upon Hewg to forge a weapon capable of slaying a god. The words "God Slain" appear when you defeat the Elden Beast. Marika betraying Maliketh by sealing destined death, to reduce the number of people who truly die, denuding the Erdtree of souls to harvest. When an Empyrean (basically one selected by the GW to carry out their will in the lands between) goes against the GW, their Empyrean Shadow (Maliketh for Marika and Blaidd for Ranni) goes mad, because they are insurance against the Empyrean straying from the designated path of control and order. Even the Night of black Knives was probably orchestrated by Marika, with Ranni's help in stealing the rune of death and creating the godslaying knives. Godwyn is referred to as Marika's unwanted child, implying that she did not want to continue her dynasty serving the GW. The Golden Order incantations all require intelligence to use, implying that Marika wants them to understand why things are happening on a deeper level, instead of placing blind faith in the two fingers/GW. Much like the glintstone sorceries, which are all derived from the amber light of falling stars that illuminate the path to deeper knowledge for those who study them. This is why in Ranni's ending, she lifts Marika's head and swears to it, instead of casting it aside and taking power for only herself. She knew Marika and had respect for what she wanted to achieve. Golden Seed description states "When the Elden Ring was shattered, these seeds flew from the Erdtree, scattering across the various lands, as if life itself knew that its end has come." Implying that the seeds are insurance against the Erdtree being destroyed and the GW's crop being spoiled. The boss rooms inside catacombs have roots that appear to be sucking bodies in.
I'm not sure how "Radagon is Marika" fits in. Granted I haven't studied all the lore yet, but one of my personal theories is that Radagon may be a manifestation of the GW himself, created for Marika to breed with, thus giving a bloodline of demigods for the GW to influence. In the beginning, Marika may not have known their true nature, but as she grew to understand it, the GW forced Radagon to fuse with Marika and exert greater control over her actions. The description for Marika's Hammer states that it is "The tool with which Marika shattered the Elden ring and Radagon attempted to repair it". Why Marika shattered the ring is pretty obvious considering the probable nature of the GW (although most people seem to think that she shattered the ring because Godwyn's death caused her unendurable grief, despite the hints that she was probably involved in the conspiracy that night), and why Radagon attempted to repair is makes sense if you assume he is an agent/manifestation of them. It would also explain why when you reach the final boss, the one hanging from the rune has a female body, then after it falls, the one who grasps the hammer and fights you is a male whose boss title is Radagon. But then after the fight, the body you approach to end the game with is female, and called Marika in the interaction prompt. Radagon doesn't want you to challenge the Elden Beast and change the course of events, as evidenced not only by the fact that he tries to stop you, but also because the Erdtree itself is inaccessible to you until you burn it with Melina in the forge of the giants. Melina was created by Marika for this purpose. When you ask her, she says "Me, I'm searching for my purpose given to me by my mother inside the Erdtree long ago, for the reason that I yet live, burned and bodyless."
Basically everyone is a puppet being manipulated by the Two Fingers and the Greater Will except Marika and perhaps Ranni. In my opinion, this is much more interesting than "Gwyn waged a war on the dragons because he just did, ok?". The dragons in Elden Ring are even given more backstory and lore themselves.
Elden Stars states "It is said that long ago, the Greater Will sent a golden star bearing a beast into the Lands Between, which would later become the Elden Ring."
The Remembrance of the Dragonlord states "The Dragonlord whose seat lies at the heart of the storm beyond time is said to have been Elden Lord in the age before the Erdtree. Once his god was fled, the lord continued to await its return."
Bolt of Gransax states "A great ancient dragon, Gransax once rained calamity upon the Royal Capital - the only time in historical record that Leyndell's walls have fallen. This marked the dawn of the war against dragons."
So Placidusax was Elden Lord before the Erdtree sprouted and Marika was chosen. After this, the dragons probably realized that the GW was about to enact their plan in the lands between, and attempted to destroy leyndell and the tree before that could happen. But Marika already had the power of the Elden Ring and defeated them.
I'm sure not all of this is correct or accurate since I'm not extremely well versed in all the lore yet, but that's partially how it's supposed to be. Miyazaki has said in the past that he leaves things deliberately vague because he likes players to fill in the blanks themselves. Personally, I'm hoping for the inevitable DLC (lol 12 million copies sold in 2 weeks) to explain the GW a bit more and shed some light on the mysteries. God damn I love Lovecraft shit.