Is there sth. like an official logo or icon for breeze?
And if not, would I rather go for the stylized plasma 5 cashew or the KDE logo?

(Background is a window watermark to style up dialog whitespace)

No, but the the new logo for Plasma is this:



There's also a plasma.svg icon in the Breeze icon theme.

Thanks, I actually had already added that in a pre-emptive strike - and because the shape is really QPainter compatible