It needs basically more than a week of extreme heat here for room temperatures to accumulate to the point that it’s crossing the mid-20s…
That hasn't been my experience. Extreme heat will do it in a couple of days, fewer if the extreme heat follows a period of normal summer heat. And that's true for well-insulated single family homes with blinders that are on the outside of windows, most of the apartment buildings I've lived in didn't have any blinders and would heat up on the first day of all-day summer sun that's followed by a mild night.
Even well-insulated homes rely on cool nights to maintain temperatures in the mid-20s for any stretch of time, and pretty much by definition, heat waves don't have cool nights.
Here in Germany, most people live in rented apartments, which makes it hard to properly install an AC unit - you'd have to get consent from your landlord, and then drop a lot of money into a space you don't own, and if you get one of those inefficient monoblock AC units, you're wasting a lot of electricity which is quite expensive in Germany. Also, before recent decades, it truly wasn't necessary; IIRC, heat days about tripled since 1990, and those 40°C heatwaves practically didn't exist.