What solutions exist for opening and closing a window very high up?
The usual approach is a "crank-type" casement window with a loop in place of the crank handle, and a hook on a pole (try "clerestory pole crank" as a search term) that engages the loop. There are also hex-type versions that use a flexible shaft inside the pole. Or, these days, motors. I'd suggest the hook on a pole, I view motors in this sort of application as things likely to fail and then make the motorized windows useless as they will probably be hard to replace when they fail. Or they'll leave you with inoperable windows during a power outage.... But it is an option.

A hook or hex style operator pole (image from BlaineWindow.com):

I can't make out how the current windows are operated - they might be able to be retrofitted with operators of that type, rather than replacing the whole window, if the only reason for replacing them would be to make them operable from below.

The style that @TDHofstetter mentions, with a catch that's operable by a hook (image from calaifornia-classics.com):