Main Street is no longer the highway that justified its current configuration. If the costs aren't significantly higher than a straight rebuild 'as is', then I support reducing the number of lanes from 6th Street to 18th Street, in order to 'reconnect' the eastern and western sections of the Overman Park neighborhood by removing the highway-level roadway that separates them.
I don't have an opinion yet on roundabouts for the 12th and 18th Street intersections. I need to know a lot more about the costs for implementation. I do see that roundabouts tend to minimize traffic by eliminating the 'platoons' of cars that all move together due to signalized intersections.
Finally, Main Street from 18th to Seerley should be reconstructed "as is". Because of zoning, this stretch of road has a very different character (commercial) from the stretch going northward from 18th (residential). I don't believe we should be entertaining lane reductions from 18th to Seerley.