Synology NAS (DS218+) and Full Disk Encryption (FileVault2)?
Every topic regarding encryption on a Synology NAS addresses “Folder
  Encryption” rather than encrypting the whole disk....As I see it, the easiest way would be to encrypt the whole disk using Apples File Vault2.


You can't use Apple's "whole disk" encryption on any NAS device because if you were actually able to, the NAS won't be able to access it.

When you are connecting to a NAS, you are mounting a shared volume (not a disk); even if that volume happens to spans the whole disk.  Synology's built in folder encryption is designed specifically to address this.