A distributed, content-addressed filesystem across the internet, not just in a home LAN, than can be mounted (fuse or whatever else), doesn’t require storing data twice, has a reasonable API and acceptable documentation.
Does something like that exist?
Hypercore’s stack has a bunch of libraries I think you will find interesting.
It emerged from the dat ecosystem ecosystem.
Note - Hypercore is now maintained by the holepunch team (hence why you will find the name holepunch in the links below)