For a “backup” (or “copy”) on an external disc/stick I would rather use rsync -av --delete ~/Pictures/ /mnt/external_disc/Pictures/…
But it should be also possible to get another systemd-Unit for a “dummy”-User that receives-only Photos from your “main” system-Unit which sends-only to this demon. GUI must be running on a different port and this unit gets a new ID. This systemd-Unit must only be started when external-disc is connected using udev-rules (or by your command).
For example:
syncthing@thomas.service and
syncthing@extern.service
🦎🖥 thomas@merz-nimbus:~ [0/1483]
09:42 $ ps -ef|grep syncthin
extern 2615 1 0 Mar28 ? 00:00:04 /usr/bin/syncthing -no-browser -no-restart -logflags=0
thomas 2621 1 0 Mar28 ? 00:00:05 /usr/bin/syncthing -no-browser -no-restart -logflags=0
thomas 2789 2621 1 Mar28 ? 00:38:26 /usr/bin/syncthing -no-browser -no-restart -logflags=0
extern 2791 2615 0 Mar28 ? 00:03:34 /usr/bin/syncthing -no-browser -no-restart -logflags=0
thomas 6530 5442 0 09:42 pts/0 00:00:00 grep --color=auto syncthin
🦎🖥 thomas@merz-nimbus:~ [0/1483]