> The untrusted device will be able to observe:
  >   File sizes
  >   Which parts of files are changed by the other devices and when

I know that cryfs[1] is resilient to at least the first of these, and possibly the second as well. I don't know if cryfs allows to modify the base directory while the filesystem is online, if it does then it might already be a better solution for syncthing, if you only care about Linux.

On the flip side syncthing could incorporate cryfs's base directory format instead of their home-grown one.

[1] https://www.cryfs.org/

edit:

https://www.cryfs.org/tutorial says the following about concurrent access to the basedir:

"Warning! Never access the file system from two devices at the same time. This can corrupt your file system. When switching devices, always make sure to stop CryFS on the first device, let Dropbox finish synchronization, and then start CryFS on the second device. There are some ideas on how future versions of CryFS could allow for concurrent access, but in the current version this is not safe."

Too bad.