There is a one way sync. So I sync from various phones to my server for Digikam / Photoprism to consume

One way sync would stop your server syncing back to the phone, but wouldn't you still need the ignoreDelete option to stop deletions on your phone syncing to your server?

You know, I think you're right. Will have to test that.

The trick is to sync to the import folder and let photoprism import them, which removes them from that folder anyways. So your phone deletes have no effect, since the photos are not in the import folder anymore. I have my imports run every 10 minutes, so there is never a huge buildup of photos to process at once. It's been working great so far

Yeah, I do move them to Digikam's folders for processing, so not a risk of actual deletion.

Syncthing can do this and much more. Its the absolute best option to sync anything anywhere. It is very flexible, uses local connections, can only sync on wlan/certain wlans. The only thing that could be better is UX - for novel users its not so easy to understand. But people hanging out on HN should choose it as its just great!

I agree it's very good, and I use it for a lot of other folders eg notes, but I don't see how to use it as a backup tool for photos, without either using the option linked above or copying/hardlinking elsewhere on the NAS end. Unless there's a setting I'm missing that stops it from deleting on the NAS end?

EDIT: Don't get me wrong, it's my favourite file syncing tool, especially as one of the few open source syncing solutions. But I worry it always gets recommended as a backup tool and people don't realise it'll also sync deletions, so they'd suffer data loss.

Oh, I do use ignoreDelete. I additionally use git-annex on the receiving side, with the daemon running, so it automatically annexes the files and pushes to long term backup that way.