Treat server storage (or even server API) as oblivious RAM
I believe I voiced similar ideas here somewhere, but I can't find it now. This is also outside of the current threat model, so maybe put it at the very end of the backlog
In short, the server operator can tell which chunks are part of the current live data, because that's the ones that the client requests every time it makes a backup. They might use this meta-data for something. I just stumbled upon a theoretical solution for this — https://en.wikipedia.org/wiki/Oblivious_RAM. The task there is to make the storage (RAM) oblivious to the access patterns of the original program; this is achieved by various schemes that hide/obfuscate the pattern by turning it into a different pattern.