Plus US Senate, federal congressional district, state assembly, state Senate, 7 judges, and 6 paragraph-long ballot propositions.
Not only your postcard idea wouldn't work, manual counting 3 pages (!) of a giant ballot would get prone to errors and be expensive rather quickly.
I think the current electronic plus storing the paper ballot for future audit if needed offers the best of both worlds.
I would expect one ballot paper per vote (based on rare occasion of doing two at once) to ensure the count is simple and accurate.
They make the ballots different colours to ensure you put them in the correct boxes iirc