Cunard are wonderful if it suits, though it's true they're a bit more "dressy" than some, so personally I'd get on ebay/Vinted or whatever for a few items which can easily be resold afterwards - unless like so many of us you get addicted, in which case you'll need them again 
They also have free self service laundrettes, so really you only need enough decent stuff for 3 days then repeat. Granted you could stick to the buffet in ultra-casuals, but you'd miss so much for the sake of spending a few quid to access the better dining
Drink prices are extortionate as are drinks packages, so if money's tight I'd stick with soft stuff just for a week, and definitely avoid ship tours if where you're going is suitable for DIY wandering (where are you going BTW?)
Gratuities are the thorny one; personally I pay them, but if you really must there's nothing to stop you asking at the purser's desk for them to be removed. It's seen as "not the thing to do" because they help to make up staff wages, but others feel that's for the cruise line to address and you'd by no means be alone
Lastly, unless it's a Southampton departure, are flights are included in the prize? Sometimes they're not so it's worth checking