Volume: System volume root's usage is 81%. This condition should not occur under normal conditions.

You can see the install history by entering cat /var/log/apt/history.log

If you enabled ssh yourself, then the most likely cause is that the previous administrator installed an app from the web UI that put data on the OS partition.  Some badly written apps do that.

Since you do care about the data on the NAS, I again suggest making a backup of the data.  USB drives are the cheapest way.  That also gives you the option of doing a factory reset - rebuilding it from scratch, recreating the shares and restoring the data from backup.  If the NAS is old, there could be some performance improvement if you do this.

You can find where the space goes by

temporarily remounting the OS partition - this lets you examine the space usage without the mount points getting in the way.  Make sure you are using "root" as the username for ssh (NOT "admin") and then enter

# mount --bind / /mnt

Then 

cd /mnt

du -csh *

That will give you a list of the space usage in each folder.  One (or perhaps two) will look really large (gigabytes instead of megabytes).

Use CD to navigate into that oversized folder, and repeat the du -csh *  The process will eventually take you to the offending subfolder(s).  The du command will then show you the size of each file - usually there are just a couple that are oversized.

What you do then depends on what those files are.  If they are logs, they can be truncated.  If the problem is that the previous admin installed packages, then they need to be uninstalled.  If they are SQL databases, then usually there is a need to uninstall one or more packages, and then delete the databases.

BTW, when you are done with /mnt, you should undo the temporary mount by entering

cd //

umount /mnt

Note that if you are the original purchaser, you can also use paid support (my.netgear.com) to take care of this for you.  However, their policy is not to provide support for used equipment.