We had 90+ degree days for 3 days in a row (and upper 80's before/after that)

Night time temperatures dropped into the 60's, so by turning on the fan at night (which blew air out and upstairs window, bringing in the cool air in throughout the house), I think we would have been fine even in a longer heat wave. By morning, it was cool enough to need a blanket in bed.

If temps didn't drop so significantly at night, then we may have had more problems.

The house is standard wood-frame construction, so doesn't have a lot of thermal mass, though it is on a concrete slab, so that may have kept the downstairs cooler.

Thermal wool has about 6 hours time lag and almost no thermal mass. The downstairs doesn't have a roof over it, so that's probably why. Typically attics/penthouses are the worst because they're sun heats them up through the roof all day long. So, white roofs, little to no thermal mass, ventilation is a solution in your area. Concrete also heats up badly and then releases the heat if it's exposed to sunshine.