No solution is perfect unfortunately. Even biometric login is easily duped, you can just hire a bunch of workers to pass the biometric check. Certainly much cheaper than bribing poor people to scan passports, eventually you'll run out of passports and we have a chance of identifying those types of accounts based on the passport country and how they behave.
"Proof of <x>hood" is explicitly a non-goal for FIDO authenticators, and implementors go to great lengths to make it impossible (primarily for privacy reasons); that's why they support anonymous attestation only, except in an enterprise context.
That's the thing about reusing existing services and protocols for "proof of x": Quite often, such proofs being possible are often adjacent to or explicitly an anti-goal.
Apple does indeed offer an API that allows uniquely identifying a given device in a privacy-preserving way, though: There's an API that allows apps to store two bits of information about a given device, such as "this device was involved in fraudulent activity on our platform" or "this device has already redeemed a free trial at one point". Not sure if Google has an equivalent.
Maybe you could require biometric login, something that requires a Google, Apple, or Yubikey issued secure enclave that authenticates on physical touch / face ID, and require that touch for every post.
For me as a person participating in online communities, I'd rather just stay in my private group chats where I have met the people I'm interacting with.