Ok but again, that is introducing an arbitrary split that is not based on tech. What if I have two phones? Would you expect phone A to see images taken by phone B? If yes, obviously my phones need to share keys.

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.