@audrew
You cannot expand the root partition.
This is a limitation due to how things were partitioned for android.

It is not safe to use apt with Ubuntu Touch as the OS is image based and updates will overwrite your modifications.

If you wish to still give it a go then simlink is the solution (or modify the mount config).
If you didn't manage to make it work by yourself, I will advise to forget about it ; I won't take responsibility for you bricking your device or loosing data, sorry.