This actually makes me think. Apple was only implementing the scanning prior to upload to iCloud, because they don't want to be liable for hosting (in any way, shape, or form) CSAM.

So in my mind, the obvious way out for everyone else is supporting things like Matrix hosting to make it turn-key for normal people. Not a managed service, but their Dendrite server and proper P2P [0] becoming usable. Now I just need to find, test for myself and family, and contribute to, a reasonable photo backup alternative (unless "Get a Synology" / "I picked up a Synology for family" becomes a crowd favourite in some insane universe).

[0] https://arewep2pyet.com/