I did look into it. By default, when the folder on the phone is set to "Send Only" and the backup device is "Receive Only", deleting photos on the phone will also delete them on the backup after syncing.
You'd have to move the files in the backup to avoid deleting or configure the backup folder advanced options to "ignoreDelete". However, not guaranteed to work. There are posts on the Syncthing forum around the same question, e.g. Send and Delete.
I'd try the "ignoreDelete" advanced option in the backup device and test if deleting files in the phone maintain them in the backup device. Note the recommendations in the documentation. For my needs I decided to manually rsync to a NAS.