Blackout curtains + closed windows do not keep my first floor anywhere near cool enough unfortunately; I think it's really poorly insulated as it is also really cold up there in the winter (though this is much more manageable to me.)

My problem with sleeping with a fan is they trigger my myofascial pain syndrome quite badly. My only relief is still air that's cooler. I also run quite hot in general :/

I have noticed, however, that I can do a single top up at around 13:00, where I run the AC for a bit to get the upstairs room slightly cooler than downstairs. Then I turn it off and close the window and it stays somewhat bearable for the rest of the day, opening the window again at night.

I completely understand your concerns about power usage as I am eco-conscious myself, and I really wish I had a ceiling fan to use instead. Getting my landlady to sign off on that is a bit unlikely.