I work on a FOSS project in this space, Blobcache.

https://github.com/blobcache/blobcache

Trusting a server to store an application's state is a different thing from trusting it to author changes or to read the data. Servers should become dumber, and clients should become smarter. When I use an app, I want the app to load E2E encrypted state from storage (possibly on another machine, possibly not owned by me) make whatever changes and produce new encrypted data to send back to the server. The server should just be trusted for durability, and to prevent unauthorized access, but not to tell the truth about doing either of those things. Blobcache provides an API to facilitate transactions on E2EE state between a dumb storage server and any smart client.

Blobcache can be installed on old hardware along with a VPN like Tailscale and then loaded up with data from other devices. Configuration is like SSH, drop a key in a configuration file to grant access. It removes most of the friction associated with consuming and producing storage as a resource.

I'm using it to build E2EE version control like Git, but for your whole home directory.

https://github.com/gotvc/got