for the dress code link from Cunard's Web site. A dinner jacket isn't necessary. Your dark suit should be fine for the formal nights. Note that many pax who choose Cunard do so for the formal atmosphere, so it's likely that there will be a number of gentlemen in tuxedos on formal nights. But, as Cunard plainly states that a "formal dark suit" is within the code, you won't be alone should you choose to go that route.
I'd love to hear your thoughts on QV when you return!