Three stops under isn’t enough. Try more like five to six and blend via luminosity masks and apply image curves. Also, shoot using a flat or neutral profile, not sure if Canon has that mode, it will give you more DR for processing. Your base ISO by default also gives you the most DR, increasing it to 400-640 only drops your DR making it harder to get detail, I shoot this type of stuff at ISO 64. Do you shoot in Raw, try Adobe Camera Raw to process the file, adjustment brush can improve blending too.