I remember reading about all of this in AP US History back in high school, and it's going to forever confound me that we seem to have uninstalled this history from our collective memory.
You should be careful about that. The College Board's APUS curriculum is heavily criticized by the americanists I know and it can be a grave error to assume that you've got the full picture from a high school textbook.
The debates are mainly about what is included and what isn’t. For example it’s very thin on Reconstruction and the end thereof; about the civil rights abuses under the New Deal; etc. There’s also debates about how to characterize certain things (for example Ronald Reagan’s rhetoric was called “bellicose” in a 2014 revision and that generated a kerfuffle.) So yes, don’t assume you learned all the history you need to know.
At the same time, don’t make the opposite mistake of assuming “everything your history teacher told you was a lie.” The New York Times made some bold assertions in the 1619 Project and has had to walk back some of the most fundamental assertions: https://www.nytimes.com/2020/10/09/opinion/nyt-1619-project-.... If you go through and strip out all the rhetoric based on factual premises that have been abandoned or debunked (even by the left, see: https://jacobinmag.com/2019/08/how-slavery-shaped-american-c...) what’s left is a really interesting take on aspects of history your AP US History curriculum doesn’t cover, but which doesn’t fundamentally contradict what you learned in school.
With respect to this particular issue: the AP US History curriculum correctly taught what the 3/5 compromise was (and it was what it was). And it also correctly taught that the Connecticut Compromise was about small states versus big states, not slavery. It was a slave-owning southern state, Virginia, that proposed proportional representation in both houses. And the small states that benefited from the structure of the Senate included both northern states and southern states.
The connection is also a weird thing to bring up today, because most of the small states that benefit from the structure of the Senate today were free states. Several, like north and South Dakota, were split up when admitted to break the power of the slave states in the Senate. Midwestern states like Iowa and Minnesota made a huge contribution to beating the south. Iowa sent troops to fight before the union army was even ready to go to war; they sent the largest percentage of their population to war of any state; and they had the highest percentage of casualties: https://valleynewstoday.com/news/local/iowa-played-a-large-r....
I appreciate your efforts here to re-expose what used to be a common understanding of this history.
As it happens, on my fraternal grandmother's side we had family that fought in the Civil War for the Union from Iowa. One day I'll look all that up and figure out what units they fought in and where they're buried.
I'm not trying to say that "the EC was about slavery" is a correct narrative or even more valid than "it was about states". The point I'm trying to make is unrelated to the specific question about the EC and slavery. The point I'm trying to make is that there are hundreds of narratives for almost every historical event. Motivations that range from the smallest microhistory to the largest historical narratives. And that "APUS said it was about X" is going to miss a whole lot of interesting scholarship.
Again, I'm not trying to make any statement about the EC. I'm trying to make a broader suggestion about the limitations of high school history classes.
