by the way I completely understand your desire to go back and do HDR. Last year I went to patagonia before I did HDR. So many lost shots
Getting it brighter wasn't really possible. It is 9 exposures already. I was shooting right into the sun which I placed behind the tree trunk. Making it brighter just blew out the sky, and the sensor wasn't able to even pick up more detail on the shaded areas. It just looked washed out. I hit the limitation of the camera.
I even shot 18 shots of this and it only looked a hair brighter but then the clouds had serious ghosting because they were moving quickly