Hey, yes this is not that difficult to do, I have done this in the past with various different NAS constellations. With Hexos you have 2 options:
If you want to backup to another Hexos or Truenas you should use ZFS replication. (this is not your usecase)
If you want to backup to a NAS which is not using ZFS you should do the backups using rsync.
For rsync you need to configure both your Synology NAS as a server and your Hexos as a client (this needs to be setup in Truenas, Hexos doesn't offer this yet).