As one who crosses Missouri several times a year from the Oklahoma-Kansas-Missouri corner to various points north and east, I would choose Bumpy's route to get to Cedar Rapids. I've found some good east-west alternatives to the Interstates, but not much in the way of north-south routes, particularly in NE Missouri and SE Iowa that move really well.
US-71 to I-35 takes you into the east side of Kansas City (you would use the east side of the loop to connect) where there will be some suburban traffic, but for Iowa destinations I see it as a necessary evil, to get to I-35.
I had been thinking of US-71 to US-54/US-24 which go diagonally towards the Mississippi at Hannibal. This goes through Lake of the Ozarks, and Hannibal is also an interesting visit, but that leaves no route north that I really like. Great River Road routing is interesting but takes you well east of Cedar Rapids. US-61 has been rough the past few times I've been on the section south of Hannibal, and carries quite a bit of truck traffic. I don't know it at all north of Hannibal.