Synology offers Hyper Backup to backup your system- and application configuration and data for a worst case scenario. You should use external storage and can use a Raspberry which runs a rsync server. The initial backup takes quite long (multiple days) but all subsequent backups will take approx 1 hour.

First you have to install rsync.

sudo apt-get install rsync

Now configure the rsync server in /etc/default/rsync:

RSYNC_ENABLE=true 
RSYNC_CONFIG_FILE=/etc/rsyncd.conf

Next add in /etc/rsyncd.conf following lines. The sample assumes the rsync server writes it's data into directory /disks/synoBackup. You have to adopt this to your local environment.

max connections = 2
log file = /var/log/rsync.log
timeout = 300
[Synology-HyperBackup]
comment = Rsync Backup für Synology-NAS
path = /disks/synoBackup 
read only = no 
list = yes 
uid = root 
gid = root 
auth users = syno
secrets file = /etc/rsyncd.secrets.syno

Now define a dedicated user for syno in /etc/rsyncd.secrets.syno (password should be a bit more complicated ...)

syno:password

Nobody should be able to read the credentials:

sudo chmod 600 /etc/rsyncd.secrets.syno

Now create the syno user on your Raspberry:

sudo adduser syno --system --group --no-create-home --shell /bin/false

On you DS create another backup task with Hyperbackup/backup task/rsync. IP address should be the IP or hostname of your Raspberry, use user syno and the password. "Backup-Module" should be "Synology-HyperBackup".

If you have any issues check /var/log/rsync.log on your Raspberry for error messages.

Security

If you have to protect your data during transfer you can select option Transfer encryption in the hyperbackup task settings. 

Performance

I didn't execute any performance measurements but I noticed following timings:

1) rsync server runs on an old Raspberry A with 2 1TB disks running in a RAID1

2) First backup run takes multiple days for 1/2 TB data on my Gb network because Raspberry is able to xfer maximum of 100Mb/s

3) All subsequent backups take between 15 and 30 minutes

4) If I have to use the backup I will unplug one of the RAID1 disks and connect it to my Syno via USB so the restore will not take multiple days.

References

These instructions were used as base. They were updated and unnessessary steps were deleted.