I know that itch.io has no plans on adding a "claim all" button on a bundle and get why.
But is it possible to implement a function to filter bundle by what you do not own? Ideally it would be what is not already claimed in the app, but I get that there is probably overlap there with why a "claim all" button isn't possible. But even just, pre-purchase, filtering what you do and do not own could help not only see the value of the bundle, but making claiming easier if you plan ahead.