That would also be a good solution, especially with multiple devices. I had tried it a couple of years ago but my NAS was a bit underpowered so I switched back to local cache.
For the OP:
Either edit advancedsettings.xml and add:
<advancedsettings>
<pathsubstitution>
<substitute>
<from>special://thumbnails </from>
<to>nfs://nas/thumbnails</to>
</substitute>
</pathsubstitution>
</advancedsettings>
or edit /etc/fstab and add something like this:
/dev/mmcblk1p1 /home/osmc/.kodi ext4 rw,relatime 0 0