I wonder if declassified cold war spy plane photos might be usable to extend the records farther back in history? The resolution and coverage should be pretty good.

They didn’t take high resolution photos everywhere back then. In the early satellites it used physical film they recovered. And later digital storage and bandwidth was expensive and they dumped any data they didn’t need.

I'm not thinking of early satellites, but rather spy planes like the U-2 and SR-71 (and their equivalents in other countries). They would take use big long reels of physical film, covering quite broad areas on continuous capture from high altitude. It's possible that much was discarded but my guess is that most of it was archived somewhere for the intelligence community. (I'm sure that some areas of the world got more attention than others, of course).