Restic seems to be a versioning / backup / deduplication archive system that could work harmoniously with Syncthing. Restic’s purpose is simple: Store many versions of a directory (or multiple directories) in it’s deduplicating repository format. It has been designed specifically to allow for parallel writes. This is possible because storing a new backup only creates new files and files are content addressed so that if two files have the same file name, they must contain the same contents, thereby avoiding possible merge conflicts. It seems then, that Restic should work well with Syncthing, even with multiple devices writing to the repo while disconnected from each other. Contrast this with git which can cause headaches or data lose if Syncthing merge conflicts happen.
I haven’t tested this yet but I am so excited about this find that I wanted to share it preemptively. I have discussed this with the Restic developer and he also thinks this should work well: https://forum.restic.net/t/restic-parallel-writes-with-syncthing-or-dropbox/797.