seafarer68, my favourite ship and welcome to Cunard. My husband wears a jacket every night, apart from Gala evenings when it's black tie but that is his preference.
You might get conflicting advice [opinions] but the truth is, on non Gala nights, a dress shirt with smart trousers fits the advised bill.
A jacket occasionally would make a nice change and many chaps do wear them but if the fuss outweighs the wearing, then don't bother.
edit
Yes, the Carnival share option gives obc on Cunard and I think it's inform between ninety days and three weeks prior to boarding.