I've accomplished the collar (easy peasy, and the twisted stitches look sharp) and the front and back panels (super-fun short rows followed by very tedious back-and-forth on a smallish number of stitches). Then I spent an hour and a half last night picking up a total of 320 stitches, along the sides of the flat pieces, and now I'm on my fourth round with a total of 460 stitches. Each round is taking forever, and there are so many stitches it's not "flowing" around the needles very well. I've already joined the shortest and the longest cables from my interchangeable needle set to make an extra-long circular, and it's not enough. The stitches bunch up in one place, so I have to stop periodically and move them around the needle.
Despite all this, it's really fun to watch the sweater come together, especially now that I'm in the striped portion. So...progress is happening, about as fast as a glacier creates a new valley.
It's a bit inscrutable in the imageabove, but the open part in the middle is in the neckhole, then there are two panels (front and back) in red, and then the striped portion is started by picking up stitches around the two panels. The striped part is shaped with decreases at the shoulder seams and increases at the four corners of the panels (shown in the two images below).