There should be no reason to setup the Drone Factory folder, either Completed Download Handling will find the paths automatically (when using a real download client) or with a blackhole client you specify the path where completed downloads go. First thing would be to remove that path, it won’t fix your problem as its permissions related, but it won’t cause other problems.
Sonarr runs under its own user on synology, its doesn’t act as guest. There are lots of posts on how to setup permissions for Sonarr on synology (https://forums.sonarr.tv/search?q=synology%20permissions), but this one says you don’t need to use the terminal:
Import Failed Error - how to resolved?