Great progress, Charles. The 12" OC stud spacing is a great way to get those walls stiff and the open rafters are perfect for ceiling refelction control. For the ceiling first reflection point, angling rigid FG board (703 or ductboard) with the low point away from the speakers between the rafters is a killer way to control reflections.
You can also use the spaces between the rafters to build 1/4 wave bass traps to control the room modes. Open rafters rock!
You are doing a very good thing here!