Well put! The flip side of this is illustrated by the ridiculous and ubiquitous scenes in movies in which our hero is running through a burning building with no protection and the fire surrounding him or her on all sides. If you’ve ever watched a large bonfire being lit, everyone stands up close, as they would at a small campfire and then have to back off to a great distance to reduce the angular size of the fire to a tolerable level so they aren’t instantly roasted like a steak on a barbecue grill.
A significant limiter to heating things with a mirror or lens setup is that as the object gets hot it begins to smolder. This smoke & vapors are at least partly opaque to the incoming radiant energy. They put a significant brake on heating your target just by shielding it from the radiant flux. This factor is one of the significant obstacles to effective military laser weapons, at least so far.
Beyond that the rising smoke also acts as a low-energy ablative heat shield, carrying away heat energy physically as the smoke drifts away from your object.
Bottom line, knock another 10-20% off your expected results to account for this issue.