As Uhtred says really, that latter is security through obscurity but I did do it for many years without any issues ... don't open port 22 though that's asking for constant attacks.
Does the synology software provide an openvpn server with cloud dynamic dns addressing (the QNAP equivalent does) which works quite well as a simple solution? I can VPN into my NAS VPN server from my phone, for instance, and then SSH into other systems once connected.