And the northern farming country would have been heavily be populated by farmers from Illinois farther north.
Changes happened over the years of course, but these three general bands largely hold up today. You see a lot more people move to KC from Nebraska than Arkansas because of cultural fit.
i agree about KC...northern missouri i'm not so sure. seems more like migration via kentucky up the missouri and overland. there's quite a few northern missouri cities named after cities in kentucky and virginia. i've never seen anything to indicate that any appreciable amount of people migrated to northern missouri from illinois (northern/central). while western illinois (between the illinois and mississippi) is somewhat like northern missouri, it changes as you head west from indianapolis towards hannibal and over to st. joseph. hannibal and louisiana, mo certainly have some southern inflected stuff going on...well ohio river/kentucky inflected stuff. sometimes when i drive between hannibal and st. joseph i sometimes forget which direction im driving...they have some similar feels going on even though st. joe is much larger.
also the germans came into st. louis very early if we are talking about the real boom years after the creole era. a much earlier period than the plains germans like in kansas or even like minneapolis.
Parts of central Missouri were indeed settled by southerners that brought their slave economy with them - https://en.wikipedia.org/wiki/Little_Dixie_(Missouri
). We typically think of the Ozarks as being the "southern" part of the state but the area around Columbia probably was, at least historically, the most southern in attitude. Even Weston was a tobacco growing area. Odd that even within the heart of little Dixie, the Missouri Tigers was a civil war era organization that was formed to fight south-leaning bushwackers. The Ozarks in the 1860's did not have a sizeable population as there was little way of making a living there. To me, "Little Dixie" doesn't really have much of a southern feel any more but the Ozarks and Boothill certainly do.
The northern part of Missouri was settled more by people from Illinois, Indiana and Ohio. The names are very anglo in nature. To find German names in Missouri, the area south and a little east of Jefferson City is replete with them: Vienna, Westphalia, Herman, Koeltztown, Freeburg. And a bit of French too - Gasconade and Vichy.
Missouri's two main cities are very midwestern and tend to go against the grain of the rest of the state. St Louis is as you describe. It was among the 5 largest cities in the US until 1920 and in the top 10 until the 60's. KC is very different as it accumulated its population at a much later date and from a different source of in-migration. I don't know many people in KC from Missouri east of KC and south of I-70. I know a ton of Kansans that have moved to the KC area but that's to be expected with KU essentially within the KC metro and the med center on the state line.