Have you already removed the built-in media (pictures, movie, music)?

Can you post the results of "dpkg -l | grep '^ii'" from your tablet, which will show all the packages you have installed? From that, we can point out the things you might not need.

Or, better yet, try this command to list all installed packages in order by size (largest last):

dpkg-query -W --showformat='${Installed-Size} ${Package}\n' | sort -n