Do the Sidereal have some form of opposition or natural enemy within the context of the lore? A metaphysical agent against the natural order of heaven as it were?
Sidereals have a lot of different supernatural forces that they have to work in opposition to. Their job, if they choose to work for the Bureau of Destiny (most do; the Bureau makes it worth their while, and it is
profoundly isolating to cut yourself off from most of the people in the world who can remember who you are), is to make sure that Destiny proceeds as planned and to like... deal with threats to Creation as needed when those two things intersect. Powerful magical beings have much more of a capacity to disrupt destiny than mortals do. Ghosts, demons, fair folk, behemoths, shit like that.
In terms of the Exalted, Lunars have been waging centuries long forever war against the successor states of the Dragon-Blooded Shogunate, and the Sidereals who back them. Bronze Faction Sidereals have also been helping Dragon-Blooded kill Lunars and the few remaining Solars for just as long. "Killed by Lunar" is the largest cause of death among Sidereals. Abyssals and Infernals are Exalt types that are typically very bad for Sidereal agendas in general, but they don't exist yet in our timeline for a couple more years.
Getimians are the closest to a spy vs spy evil opposite, in that they're like, weird paradox people formed out of unrealised destinies, who are created in the moment of their Exaltation, but who remember an alternate timeline where they existed and did great and heroic things. So unlike a Sidereal, who is real and who everyone simply forgot, a Getimian just like... gets dropped into a slightly-wrong world where they might meet people who they remember, but none of their memories actually happened so everyone is different and has no idea who they are. They have weird powers that fuck with fate and destiny, and a bunch of them are actively working against heaven.
Sidereals have much better odds of living a long life than a lot of young Exalts, though. Heaven's support and training is a lot, and they have abilities that let them move through the world really well. They can live for thousands of years, if nothing kills them early.