Another way to create symbolic links...from the command line:
ln -s [folder to be linked to] [link location]
So in your case...
ln -s /mnt/sdb#/documents /home/documents
-or-
ln -s /run/media/<user>/deviceid/documents /home/documents
Note: You'd need to delete the documents folder under /home if it exists prior to creating the soft link.
Device ID can be obtained from DoubleCommander (should be installed if you're using Budgie) or GParted.
ln...as in the letter L and N, but lowercase. ;-)
Your solution (using a microSD card) is doable, but I'm thinking you'll quickly wear out the card writing to and deleting from it so frequently. Why not buy yourself a portable HDD/SSD?