The principal works of Kant, On The Fourfold Root of the Principle of Sufficient Reason, and The World as Will and Representation twice, which I am sure most Schopenhauerians have read
Bruh, what 18 year old is gonna decide to read Aquinas, Aristotle, or Kant out of the blue,
Unless they already had some familiarity with philosophy.
Nietzsche and Marx have a familiarity in modern literary culture.
You ask someone who's read a lot of philosophy a question and they say "Kant said this about that..." ok but like this is also what you believe, or....?
Most things people consider to be ancient traditions inherent to their cultural identity were really only created by some freaks in the 1800s, and the ones that actually are older usually suck