Your "Driving Game" mockup got me thinking about indicating proportions on the drafts. I don't have a good answer to that, except that Cards can be resized in width. At any rate, to answer "perhaps it is slightly too high fidelity", I would say it's the opposite, too low since there's no layout design involved.

I'm aware it‘s confusing because Cards look like a mobile layout. I tried doing the pencil sketch style but it also gave the impression of a layout design tool.