Sure, I can see that. But now we are left with another problem: if there is a central authority issuing your keys, how can you be certain that the keys are safe?

I’m not against the idea, but if the “solution” requires some form of central authority to verify your identity, then you don’t need to cryptographically sign anything. Show up, prove your identity, put a ballot in a box.

The benefit of cryptographically signing a ballot would probably be a verifiable ledger (which could be verifiable by (nostr?)relays).

I might have misunderstood what you meant by authority. Another solution I just thought of could be that you make your own keys (no central issuer), an authority verifies your identity and logs it. If you show up again to vote with another set of keys the person verifying your identity would stop you. In this scenario the ledger and the keys would not be controlled by an authority. But there’s still some trust involved; you would have to trust that the central authority itself isn’t cheating.