Hey there,
I’ve been playing around with the snap installation all day so far and I’ve got something similar working. Go to this link (https://askubuntu.com/questions/882625/nextcloud-snap-with-data-directory-on-external-harddrive) and follow the instructions to the letter (apart from changing your data directory). In general, snap packages can’t access folders outside of their directories, so just mount your directory somewhere in /media/… and the instructions should work. As I say, I’ve tried lots of different workarounds, but having to mount in the media path is the only thing that’s worked for me - and it’s actually usually only a small price to pay.
Hope this helps (someone),
clarktw