I am not suggesting to not use a VPN. But a VPN alone doesn't solve the visibility problem. Similar to port knocking, this would prevent anyone from knowing tha

I am not suggesting to not use a VPN. But a VPN alone doesn't solve the visibility problem. Similar to port knocking, this would prevent anyone from knowing that the server even exists without knowing the IP and key in advance. This isn't an alternative to VPNs. In fact, as far as I'm concerned, this proposal is only useful when combined with a VPN!