What actually happens if you decode a JWT's payload without a library — is it 'safe' to just read?

Yes and no: the payload is only Base64-encoded, not encrypted, so anyone can decode and read its claims without the secret key — never put sensitive data like passwords or PII directly in a JWT payload. What the signature protects against is tampering: changing a claim (like a role) invalidates the signature, so the server will reject a modified token even though the attacker could read the original.