Hey there, I was looking for a backup solution in syncthing and as feature-rich it is I did manage to make it work.
However, there needs to be a backup folder as there is in resilio. Where if you select that option the sync will happen one way and deletes on the main device will be ignored.
To achieve this in syncthing, one has to add IgnoreDelets and the android app is not very friendly when it comes to input patterns I had to switch to web GUI on the android app for a better experience.
With all the things syncthing can achieve, I can’t believe there is no straightforward way to backup a folder.
I have moved on to resilio as that was what I needed, hope to see this in the future.