And to avoid mishaps, you can also set the phone side to be send only.

Do remember that as you delete photos on your phone to save space, it will delete them on the PC. You would need the PC side to move the photos to a different directory (a cronjob would do).

Actually you can setup syncthing not to delete the file in the receiving folder https://docs.syncthing.net/advanced/folder-ignoredelete.html