Well, I made my way to the US now, and we drove to Yosemite. We had taken my lightest possible imaging gear: light photo tripod, AZ-GTi mount, Canon 200mm f2.8 lens, and ASI294MC Pro. We were quite busy so I had very little time yesterday evening, but I managed to get just 45 minutes of data (140x20s) in AltAz, from a meadow not far from the Upper Pines camping ground. From one used to the light pollution in Tokyo, the result is amazing!
So it seems that Alt-Az is a perfectly decent way to get data with such a fast sensor, which is good news!