We have done one, back in 2016, and whole I had a tuxedo with me, formal night was not as formal as I expected. Yes many men were in tuxedos 30%, many more in dark suits 50% but also many more with not even a black jacket 20%.
The women were similarly dressed, ball gowns, cocktail dresses, evening chic, there was a lot more variety and a bit of being went a long way to fancy up their outfits.
I wouldn't worry too much, we had one man onboard that never got out of shorts for the entire cruise, the buffet was good enough for him (it was good). And another man at our table had a tracksuit jacket for his jacket, wore it into the dining room and then hung it over his chair.
We do have another cunard cruise in December.