Not sure I would have taken M-21 all the way to Lowell since it is about as straight as a road gets and plenty of small little towns to slow you down. But I get being unsure of your bike on the freeway.
I-94 tends to be cluttered with semi's. They're all trying to avoid the tolls of I-80. As such, I-94 tends to be rough with heavy traffic. Have to be on your game there. I-69 is usually a breeze though. Sadly in Michigan, the freeways are about as twisty as the roads get. 200 miles is a good size day, glad it worked out for you.