My wife greatly enjoys the formal nights and I'm OK with it a few times a year. We sailed the QM2 on a TA back in May (7 nights). The three formal nights were just enough. Not sure I could have done 6 on a two week voyage as you are talking about. However, Cunard and the crew of the QM2 certainly made those 3 nights special and a lot of fun as did our fellow passengers. Casual nights are very easy. For men it is just a blazer. For women, even easier as cocktail party styles were quite diverse. Daytime attire was again all over the board with the exception of "shabby chic". Didn't see any of that.
We had a couple of chilly days but managed to still get our walks in after breakfast on the promenade deck. We also found the hot tubs on the stern of the ship (deck 8 I think) were great in the afternoons on those chilly days. We would head out there in our robes with our bathing suites underneath and a couple of large glasses of wine.
Still, going on a 14 day cruise on a Cunard ship when formal is not your thing may not be a wise thing or as my old boss use to say, "bad plan executed perfectly".
Hint: A number of years ago I bought a used tux from the rental department of the Men's Wearhouse. I picked a classic design and the sales clerk let me go through the inventory until I found pants and a coat in very good condition. The cost was about the price of 2 1/2 rentals and that included a final dry-cleaning. I have worn that tux more than a dozen times since. For our cruise this year I bought a second tuxedo shirt with an updated look.