Attire on cruise

I think you'd receive more feedback if you posted your question on Cruise Critic. That's a forum for all cruise- related issues.
Have a great trip!

Button down shirt! That’s a No no. No one uses the buttons anymore.

Personally I don't bother with formal night- to me that is a very good night to go to the buffet or one of the other casual dining venues.
The only line left with rigid dress codes is Cunard (why I wouldn't sail with them if you paid me to) and even they are starting to relax things a bit.
I sail on an informal line, but even there have encountered the cruise snobs who are stuck in Downton Abbey times, sailing 1st class on the Titanic.
Sneering, look down their nose types if you don't comply exactly with what they think is right.
That is exactly what you'll get on Cruise Critic, in my experience.

Looking at the RCL dress code is all I would do. Many ladies, and the RCL code confirms, would wear a nice dress, men a tie (not bow tie) and (in that climate) a lightweight jacket. To be honest, for you, I can't see why the slacks wouldn't pass muster.
If the Maitre d thinks he is working in the Savoy and turns you away go somewhere else. Life's too short. It's a vacation.
I've only gone over the top once, for Captains Table- when I was in as much Highland Dress as I could muster, minus the kilt.

I don’t sail Royal Caribbean and if I did, I’d be eating in the buffet on any formal night.

However, I suggest you check out some reviews of recent cruises on Youtube

Have never sailed with Royal Caribbean, so I cannot advise re: what to expect on that large ship. But, on our first Silversea (high-end, small ship), the line recommended formal dress (even tux) for the "formal night(s)" at that time.

Sooooo, we dragged along my husband's tux, dress shoes, etc., and I took a nice cocktail type dress.

NEVER AGAIN.....what played out? Other than very few exceptions, the only other guys in tux attire were the waiters!! What a laugh we got. Sports coats/dinner jackets were what MOST of the men wore, and some not even that.

I do not even recall a formal night on our subsequent SilverSea cruises (maybe we just enjoyed cabin service or my husband just worse a sports coat (and fit in nicely). High end or budget....travelers generally want comfort.

Other than expedition type cruises with National Geo (where a clean flannel shirt is considered appropriate for dinner), we have not cruised in years, and it seems to be moving toward a more relaxed mode on even the high-end cruise lines.

I would suggest calling your cruise line and asking.

But, personally, if it is only one "dress night," we'd rather just enjoy room service that night (or as another poster mentioned, the buffet, if available as an option) vs. having to lug all the extra clothing/shoes. Granted, if we had a dinner opportunity to meet the King of England or other, yep, we'd drag the attire...otherwise, nada.

Enjoy your cruise. Be comfortable. Life is short.

OP, what you described would be the minimum acceptable dress for a formal night on RC . And no sneakers. Depending on the cruise, you may see quite a few or very few in full formal wear. Most will fall somewhere in between. I'm linking an article from Emma Cruises, a well known cruise blogger/vlogger, which may be helpful: https://emmacruises.com/royal-caribbean-dress-code/

Yes, your planned dinner dress for formal night is fine. You don’t need to dress up for RC formal night to eat in the main dining room. I just wear black pants and a with a simple top (often black with costume jewelry). All black tennis shoes finish the outfit. My husband wears slacks, collared golf shirt and whatever tennis shoes he is traveling with. We just made the transatlantic sailing this spring. There were people less dressed up and others in their cocktail dress and high heels. We have cruised for 30 years…early on we dressed up formally but have simplified our travel clothes dramatically. I like slow dining so dinner in the buffet is not an option.

The only line left with rigid dress codes is Cunard (why I wouldn't sail with them if you paid me to) and even they are starting to relax things a bit.

I went on my first, and probably last cruise, with Cunard last year. Whilst I did smarten up for Gala Night it was with a suit and tie not a tuxedo. That was in Queens Grill and there were people who were dressed far less formally than I was so if Cunard are fine with less formal attire then I would imagine that on Royal Caribbean it would be perfectly acceptable to wear what the OP has described.

I would like to wear a nice pair black slacks and shimmer bksvk top and husband wears slacks and button down collar shirt. No jacket. Would this be acceptable?

I'm with @jojo biker in saying you will be fine wearing what you are thinking. RC will definitely have a "more casual" formal night dress code due to the nature of the line and the many families on board, and I have found warmer "more summery" cruises in the Mediterranean or Caribbean will be the most casual of all types of cruises.

In 2025, with the exception of the more strict (eg Cunard) lines, formal night in the main dining room (MDR) will usually be "most people trying to dress well/appropriate" but more than a few folks will be in the "I don't care what the rules/expectations are" category. Add in RC as a family cruise line, and you'll always have folks dressed FAR more casually than you on any given evening.