The short version: Historic Venetian costume and mask if you are going all in; otherwise a mask over dark tailoring.
The dominant aesthetics at Venice Carnival are carnival costume, full costume, quiet elegance and gothic. That is the visual language of the site, the stages, the merch, the crowd photos, and outfits that speak it look intentional rather than borrowed.
This is a costume festival, and that changes the maths: turning up in normal clothes is the conspicuous choice. Budget for one full look rather than three half-hearted ones, and make sure you can walk, dance and queue in it for eight hours.
What people actually wear: a real coat over the outfit, thermals underneath, and boots with grip; something identifiable, a colour, a hat, a flag, so your group can find you in a crowd of that size.