This took way too long for me to find a solution for, but it’s all good now. Apparently Seafile doesn’t like symlinks!
Instead of symlinking the my mounted /media/nas/seafile directory to /opt/seafile-data/seafile/seafile-data, I just mounted the drive directly onto the seafile-data directory itself!
Such a painful journey for such a simple solution.
I discovered this method from a previous topic on the forum: