When I type @due I’d expect to get the “insert date” dialog as seen on the screencasts but it doesn’t appear. It appears when I use the “Tag with” shortcut and select “due” then the popup appears.
Should I press any combination to get the panel as an autocomplete when y type “@due”?
It’s not intended to show when you type out @due. That feature has been requested, but not implemented yet, and not sure that I will. In the meantime I recommend that you just assign a keyboard shortcut to the Tag > Due menu item. That will enable you to tag items due in a single shortcut.
Either TextExpander or a custom script would work I think. You’d get more flexibility from a script, but if your needs are just text expansion, something like TextExpander would be easier.