To bad you can't just cancel Auto Steer and leave TACC on (smoothly).
you can! Just put turn signal on while autosteer is enabled. It'll do just that, disable autosteer, and leave TACC running. The only times i've seen it not smooth is if there is another car in the next lane while trying to do the lane change, so it'll brake while in the middle of changing lane. But that's to be expected. But as long as lanes are clear, TACC won't do anything abruptly. I've been doing it this way for a while now, and I think TACC behaves just fine during lane changes