It should be noted that the United States at the time of the drafting of the Constitution had way way lower rates of education than today. Even as recently as 1945, the median American only had a 10th grade education. No matter how you measure it, literacy, primary school completion, even intelligence tests, there's no question that the Americans today are significantly more educated than a hundred years ago let alone two hundred.
Yes, I know you explicitly said not to focus on "technical" education. But there's no reasonable definition of the term, whereby you can honestly make a case that the America of 1789 was somehow more educated than the the America of 2019.
All of which means that you have to bite a bullet. Either the Founding Fathers were wrong to enshrine such strong protections of free speech in our Constitution. OR admit that our citizenry more than meets the threshold you posit regarding minimum level of education.