When I ran the native Duplicati package on my Synology, I would use a symlink. First, stop the Duplicati package so it is not running. Then ssh to your NAS as root and do something like this:
# cd /root/.config
# ln -s /volume1/backupconfigfolder Duplicati
You may need to remove any existing Duplicati folder before you can create the symlink. Use caution so you don’t delete anything important.
Other thing to note - Duplicati 2.0.5.x requires Mono 5.x or newer. You cannot use the Mono package provided by the standard Synology package repository as it’s too old. You should remove that Mono package and install the one from https://synocommunity.com/
Once you have the newer Mono version installed, restart Duplicati package and it should use the correct data folder pointed to by your symlink.
This symlink may need to be recreated every now and then. Major DSM upgrades like to wipe out the /root/.config area.