You are using an incorrect location of the configuration file. It’s located at /mnt/dietpi_userdata/filebrowser/filebrowser.db as described on our online docs
Other than that, following your steps, you will lock File Browser to /mnt/mycloud/FilebrowserDat. It’s not able to store data outside of this directory. In File Browser this will be shown as your root directory.
Yes, it will set the path what will be shown in file browser to /mnt/mycloud/FilebrowserData but the config file of file browser still remains at /mnt/dietpi_userdata/filebrowser/filebrowser.db.
You could move this config file, but then you would also need to modify the according systemd service, so file browser will load the config from a new path.