They are all valid options, but I feel like it defeats the purpose to rely on and have to trust third party commercial services with your data.
Otherwise, you might aswell just provide a service like facebook, or whatever else you're trying to replace, and offer people to encrypt their files on upload, if they do not with them to be public.
The pain point of your solution is data security. Not in the sense of it being vulnerable to surveillance or digital theft, but what happens if someone breaks in and steals your little black box, or if the house burns down?
Syncing it to a CloudBased service kind of breaks the concept.
One possible solution I can think of is to allow people to store encrypted copies of oneanothers black boxes.
Imagine that your blackbox would automaticly sync all it's data to my blackbox where it was encrypted with a key that only you had - and of course I would do the same the other way.
That solution is, however, only possible if all partipitants have really solid internet connections.
Otherwise, you might aswell just provide a service like facebook, or whatever else you're trying to replace, and offer people to encrypt their files on upload, if they do not with them to be public.
But I might just have missed the idea :-)