You could use Syncthing just to empty the incoming files from your phone (ingest) and then move the photos via cron to a second folder (also Syncthing) which is just shared with the replicas.

Another approach would be to push the files from Syncthing to borg (borgmatic can do replicas) https://torsion.org/borgmatic/

I import files from Syncthing folders into a git-annex on my NAS, where multiple copies are eventually guaranteed via sync to off-site mirrors (remotes).