Once the sunlight has passed through the window, the battle against the heat is mostly lost. Awnings, external shutters and to some extent solar protection film are some of the better options in this instance.

These are things we've had the luxury of not having to care about in the past, instead being able to harvest some solar heat during the seasons where heat is scarce. Less so these days.

In any case, I think "our homes are not designed to keep heat out" would be an accurate statement for northern Europe.