i really wouldn't be worried about being overdressed on a Cunard cruise.
sounds like you have formal figured out, and you are right, formal is easy.
for Informal - men - jacket & tie, ladies - nice dinner dress or dinner suit, dress trousers with a nice blouse.
Casual - men - dockers type casual pants with collared or buttondown shirt. Ladies - again a nice day dress, skirt/blouse, trousers with a nice blouse, sweater twinsets paired up with a nice pair of trousers are great for casual nights.