Any chance of building in functionality to have cloud based storage backend (eg some server running sshd).
Theorectically could cobble something together with WinFSP but would be nicer to be embedded functionality.
If it works, then can have the software server doing all the dedup / syncing but not limited to the storage on computer.
I think that is what nextcloud, pydio, own cloud etc. are for.
Syncthing is targeted at syncing local storage, between various devices anywhere in a network. Reasonably fast filesystem access is required, or is at least important enough to cause rather low performance otherwise. To me pointing a Syncthing folder path at some central cloud storage is somewhat antithetical to its core values (e.g. being decentralized). Running Syncthing itself on a cloud storage provider is already better aligned, there the question of privacy remains.
And then comes the “do one thing and do it will” argument: Accessing a remote filesystem is not Syncthing’s purpose, it’s syncing local data with other Syncthing (BEP) clients. Using some other tool that is purpose built for accessing remote storage (in your case a dedicated ssh client) is good.