> Unfortunately, what�s missing is provision for unmounting the stick when the backup is complete, and some way of notifying the user that it is safe to remove it.

In a non-GUI environment, then I guess if they install "Policy Mount"...

sudo apt install pmount

...then use the pumount command line utility of the pmount application. The pumount command could be invoked with a bash script to do the unmounting of the USB drive without requiring priv.

I recently opted for...

https://github.com/raamsri/automount-usb

...as a better tool for mounting USB drives over...

usbmount - automatically mount and unmount USB mass storage devices

I combine automount-usb with pumount to do both the automatic mounting and manual unmounting of USB drives on a Debian server.

cheers,
Ian.