Nice slacks are fine and apparently, it seems a polo shirt is acceptable for dinner of an evening onboard but I haven't seen a polo shirt in the [QG] restaurant over dinner [lunch, yes] or if I have, it was wearing a jacket, and so in my experience, I haven't seen one.
However, if it's allowed, it's allowed. 🙂
For info., most chaps will go the long sleeved shirt route for dinner and at least half will wear a jacket too. A smaller minority will also wear a tie.
For Gala evenings, you're sorted with the black suit but again, more will be in DJs than not.
Enjoy, whatever you wear. 😃