Shared-by-default is the right posture for a household system. The calendar, the lists, the chores — the value is in everybody seeing the same thing. And then once a year that default is exactly wrong.
Privacy inside a household is real
It is easy to treat a family as a single trust boundary. It is not one. Gifts are the obvious case, but they are not the only one: medical appointments, financial documents, a private note about a difficult conversation. A system that cannot keep something from one member is not finished.
Which is an architecture question, not a setting
Hiding a row in the interface is not privacy — the data is still there, one bug or one API call away. Real per-member privacy has to be enforced where the data is stored, so that the answer to "can this person read this row" is decided by the database rather than by the screen that happened to render it.