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.
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 doesn’t work like that as that’s not the intended purpose of Syncthing. It’s a drive/folder mirroring software not a backup solution.
I wanted to use it to backup photos from my phone into a hard drive. When the photos are deleted on my phone, Syncthing sends a delete command. The send and receive options in Syncthing are for commands but delete is also a command that is sent.
Rsync is likely a solution to incremental copy of files from a phone to a NAS where you can delete locally without impacting the NAS backup.
Cryptomator?