1. While simple TCP/IP implementations have SYN flood issues, there are plenty of ways to mitigate them, including SYN cookies and third-party firewalls and load balancers. In fact, the SYN flood mitigations are so good, that most modern attacks are either raw traffic, or higher-level connections.
2. TLS pads your messages to block size (for example, 16 bytes). If your message size varies by a bigger amount, just pad all of your messages manually. This takes 2 lines in modern scripting languages. No need to switch to all-new suite.
3. This is just outright wrong. If you have messages which are "fully encrypted, authenticated, validated", then it means you run ZeroMQ with either TLS or CurveZMQ. Both of them have full protection against replay attack.