So what is different in saying „All family devices should see images taken by any device“? They are all clients, including the storage&processing device.
Insisting that E2EE have „ends“ on clients is not useful. It’s much better to define „end“ based on control.
Here’s Wikipedia’s definition:
„End-to-end encryption (E2EE) is a method of implementing a secure communication system where only the sender and intended recipient can read the messages“
Note the complete lack of client/server distinction. It’s simply about intended recipients.
E2ee is quite well established imho. The server at Hetzner (a.k.a. "someone else's computer") is NOT to be trusted in e2ee schemes. Until today I'd be willing to say everyone agrees on this, but now I doubt that :)