I think you might feel more comfortable on Azamara or Oceania than Regent or Seabourn.
That said, “dressing up “ on Regent can be as simple as slacks and a collared shirt (cleaned and pressed with the free laundry)/ dress or skirt and top, for dinner.
If that is too much for you to enjoy a “laid back vacation”, Regent is probably not right for you.
IMHO, I, for one, believe we all should care, at least a little, about what others think. Otherwise there are no norms of behavior.