Within the context of Arknights describing its empires in the same terms it describes its supernatural entities and how it extends the concept of assimilation from being digitalized into pure information by magical rocks to being absorbed into pseudo hive minds to plain old nationalisms, I believe the Bestial Sui is best read as a metaphorical mirror to the True Lung and the nation of Yan itself.
We always knew that Sui was yet another expression of the recurring theme of assimilation, a constant threat to the Twelve Proxies' individuality, who would all stop being themselves and lose their humanity the moment It awakened; the latest event went all in on the horror aspect of it and the fear of everything you are being consumed, like with the word "Sui" in red taking the place of the names of the siblings in Wang's vision and expanding over the pages as he was trying his hardest to delete it. The great focus each of the siblings had on culture and different forms of art and in general following their own passions and building their distinct identity for centuries is very similar to how artistic and greater meaning and personal goals are emphatized in the Iberia and Aegir storyline when it comes to resist being fully assimilated into the Seaborn.
But in First of a Thousand Autumns we learn two new things:
Sui sees itself not merely as the ruler of this land but also as a guide, intentioned to lead humanity in its journey to escape ruination and reach the next stage, which puts it in a similar category compared to a lot of the other self-centered yet idealistic antagonists in the game (from Priestess to the Witch King to the Church of the Deep and even fervent nationalists like the Emperor's Blade);
Sui would be empowered specifically by the personal strength the Proxies cultivated all this time.
Fundamentally this is just the mythological version of what the previous True Lung and the Gran Commandant talk about when they define Yan's expansion and growth not only as natural and necessary but even beneficial because as the greatest nation it's Great Yan's duty to lead everyone on this land to prosperity and it doesn't matter how many are sacrificed in the process as it's only natural that they would give their lives to fuel Yan. Is the way Sui treats its Proxies really all that different from how the True Lung treated them? If the nation reached this level of prosperity it's also thanks to everything the siblings did, all their efforts and the talents they cultivated and offered to it for a thousand years, and yet the True Lung was fully ready to just discard the twelve if it was for the sake of even greater prosperity than what they already had, because the idea of Yan as a whole matters more than any of its parts. Just like Sui, the empire doesn't care about who you are as a person; it will simply reap the fruits of your work for itself, discard you when you're no longer needed, and continue on its way, acting as if you never existed.
A major focus of the event is in regards to history, and in particular it puts heavy emphasis on never forgetting the sacrifices made to get where you are. Jie is seen as the single most dangerous proxy entirely because she doesn't allow the truth to be obfuscated; she isn't even using her power to alter history but just to make it so it can't be falsified, and that's enough to threaten the government's authority. It's all about never forgetting the people, the individuals sacrificed for Yan's advancements, no matter if they were necessary or not, and that's reflected in the proxies' fear of vanishing as if their existence was just Sui's dream. She's the one most resisting the assimilation of the yanese people into the singular force that is Yan, and similarly she's the one who - alongside Wang - most resisted the assimilation of the siblings. Both the citizens and the proxies are always at risk of having their existence and sacrifices erased.
Both Sui and Yan alike fear Jie growing in power and eventually stealing their absolute authority. When people start talking about Jie potentially becoming a threat superior even to the original Sui, the way that fear is expressed in the terms of how she would gain absolute power over the written word of Yan, to rule over the state; something not any different than what Yan was already trying to do. The closer she gets to being assimilated into Sui, the more she'd start using her powers to impose her will and authority over others rather than simply supporting them as she's been doing until now.
To become Sui is explicitly to become the ruler of Yan, at the cost of making that your entire identity; to reject it is to keep living as a human.
The way Wang describes Sui, its arrogance and loneliness, is it all that different from Blaze's criticism of the current True Lung in Such is the Joy of Our Reunion? Someone cast in the role of a supreme ruler, with no one to be its equal, who is not allowed to be wrong, to be themselves, to be human.
The crucial difference between the Proxies and their parent is that they walk among humanity and support them. Sui wasn't wrong for wanting humanity to reach the stars, it was wrong for thinking that it could only be done by ruling over them as a king. Sui mocks their way of life as "hiding their claws" (for as much as it may be motivated by necessity) because it can't understand living without imposing your will on others, as that's the only way someone born as a powerful ruler could interact with this land.
Both in regards to Sui and True Lung the story remarks that such a figure is not needed - whether it's an absolute god or an absolute emperor.
That's also why there's such emphasis on how just killing Sui would be useless because another proxy would take its place. It's the same as killing the emperor; someone else would simply become the True Lung. You can see that even as Wang became Sui: the old one would continue to torment him, to torment whoever sits on the throne and slowly turn them into it.
Wang has to completely remove Sui from the equation, to have it be consumed by the Originium for eternity; he had to make it so that the Sui could never again harm anyone and no one else could ever become it. Crucially, in doing so, he also removes the piece of the first Originium that is said to be the source of the True Lung's power and authority, the one that allowed him to replace Sui as the ruler of Yan.
...Which is kind of the high stakes mythological world-shattering version of Ch'en hoping The True Lung's daughter will be able to change this system altogether rather than simply take her father's place.
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