I like the idea of the floral dress, plain cardigan and black tights. I would add a wedge heel rather than flat though. A nice top under the cardigan in case it is hot weather, one that doesn't show if you are hot (I perspire under my arms and find it embarrassing). If it is hot, perhaps footless leggings with a nice wedge sandal even.

A top from Metallicus would go nicely with a floral dress..they are light to wear, versatile and last a long time. Then if it is hot, you don't need the cardigan. The Metallicus tops come long sleeve and are comfortable to wear in summer. They are also a one size fit all stretchy material that allow for 'those' days

ETA: Dress...still could add a top rather than cardigan maybe over top, depends on cut of dress???