The location of the configuration file

Karabiner-Elements stores configuration to a json file which is located ~/.config/karabiner/karabiner.json

If you want to move karabiner.json to another place and make symbolic link, make a symbolic link to ~/.config/karabiner directory instead of karabiner.json.

The following command allows you to put karabiner.json on ~/Dropbox/private.

mv ~/.config/karabiner ~/Dropbox/private
ln -s ~/Dropbox/private/karabiner ~/.config