As you only have around 500MB free on your internal storage I think you need to start by figuring out what it taking that room, and do what you must to free up some free space. As your Thumbnail folder for your master user looks to be an appropriate size you will have to do some digging around yourself to find where the space is being used. You can use that command on other folders just replacing the file path such as du -sh ~/.kodi (this would be how much room all your userdata is using), or alternatively you can use the file manager in Kodi and just context menu>calculate usage on them.
The files I recommend removing from the NAS is only the artwork. I’m sure one of the Linux people here could give you a safe method of doing this via the terminal, but since i’m primarily a Windows user what I recommend is browsing the NAS to your media folders (in Windows) and search for *.jpg and once the search result finish populating ctrl-a to select everything, right click on a file, and select delete.
To manually remove your cache…
systemctl stop mediacenter
rm -r ~/.kodi/userdata/Thumbnails
rm ~/.kodi/userdata/Database/Textures13.db
systemctl start mediacenter
Note that this only applies to the master user. If you are using profiles then they have to be taken care of seperately.