Hmm. First backup your entire N900.
You probably only need to mess with rootfs & not optfs.
rootfs is at / downwards. optfs is at /home, & /opt downwards.
When you install apps, many of them go under /usr/lib, /usr/bin & /usr/share. Try temporarily moving only app specific folders, new and large directories from those locations to say a temporary /opt/z directory. Do this until you have space to install CSSU thumb.

You can ignore this if you're not familiar.

kh