There have been a bunch of projects that tried to do some variant this, and I'd love for it to exist, but I'd almost posit it's an impossible problem. Projects either find a way to handle the content-addressing, but then fail on coordinating nodes, or can coordinate but can't choose placement efficiently, or are just vaporware. I think the hard part is most personal computers are too unreliable to trust, and centralization, even for a homelab experimental user, is just too easy.

A few projects that tackled some version of this...

Nextcloud, Owncloud, and generally just NAS can be "dropbox but self hosted" but its centralized.

IPFS, Perkeep, Iroh, hypercore (npm), focused on content-addressed information, making cataloging and sharing easy, but fail to really handle coordination of which node the data goes on.

Syncthing, Garage, and of course BitTorrent, and a few others can coordinate but they all duplicate everything everywhere.

"Bazil.org", and (dead link) "infinit.sh" both sought to coordinate distribution and somehow catalog the data, but they both seem to have died without achieving their goals. I used infinit.sh when it was alive ~2016 but it was too slow to use for anything.

I'd love for something like this to exist, but I think its an impossible mission.