I would like to have in place of the Hero banner, a fully portrait mode slideshow.
Here is an example: https://www.chouetteclub.com/
therefore, when on desktop etc, you auto see 3+ images rotating.. and on mobile it's already fit to screen. Most of my visitors are mobile based, and the difference/ difficulty to find an appropriate image for both landscape (desktop) and portrait (mobile) often means sacrificing my best content.
is it possible with Brooklyn?
Brooklyn theme by default provide a banner slider. From Dashboard->Online store-> Customize you can assign images to slider and then can change setting to auto-rotate slide and set the seconds after which it should be rotated.