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.