A break in the weather and finally able to test out my workflow changes. Turns out the camera built-in noise reduction really needs to be enabled for this kind of photography. In this case, both long exposure (dark frame subtraction) and a new enhanced high ISO speed noise reduction is enabled. Successfully eliminated the mysterious banding I was seeing in post-processing.
This image was taken with 180 x 15 sec. JPEG frames at 3200 ISO, field flattener, strong noise reduction, and of course, unguided.