You or anyone else can arrange to ship luggage to the ship. Seabourn has this service and there are some independent companies that do it as well. It isn't cheap, though. I also wonder why people don't just do a quick shop before they board the cruise as it can be less expensive than the extra luggage charges. But I am a guy.
There really is more to a formal evening than the clothes. On Seabourn you may be greeted by a 6 to 9 course Chef's Menu with elegant cuisine...and not a show with frozen lobster tails with people trying to consume as many as possible shrimp cocktails as possible. That isn't what happens in Grill, but in Britannia it is, alas, a different story (as it is on many more mainstream lines). When I was on QM2 the Black & White Ball was fairly poorly attended and there were more suits than tuxes. It varies from cruise to cruise.
There is no doubt the Cunard is a favorite for some, Regent is for others and Seabourn is for yet others. It is, alas, whatever floats an individual's boat. That is why it is important for folks to get information - from places like here and knowledgeable travel agents - so that they have the cruise experience they personally want and not what is perceived from brochures.
Just as clothes don't make the man/woman formal nights don't make the cruise's dining experience formal...high quality...enjoyable.