To the person who suggested putting an emergency mylar blanket over the window (my apologies, can't find you comment anymore), wow that worked well. The indoor temp dropped 4C in 15 minutes, and that was with no direct light aimed at the window. I'm surprised that extent of solar energy was coming through. Nice hack, and one that is easy/cheap to do.
Edited to add: I always thought those blankets are opaque, so expected the effect to be like a blackout curtain. But it turned out they are more like a tinted car window. The amount of light is notably reduced, and the residual light bleeds through with an orange tint. But when there's light outside as backlight, the view across the street is still clear and detailed -- just darker. At night it's effectively opaque. (Though don't make any assumptions re privacy since now the interior is what would be backlit.)