To add to this, while as you say, it's
possible flat earth might have been a thing amongst the illiterate, we have lots and lots of evidence that shows that the idea of a spherical earth had not been forgotten in Europe at the time either (with numerous older sources, but just to specifically address "600 years ago...):
Dante's Divine Comedy, finished in 1320, for example, goes into a lot of detail about the difference between the hemispheres such as how it affects which stars are visible and the concept of time difference and where he imagines the sun rises and sets over different parts of the world, and how gravity changes as Dante travels through the earth to arrive at other side.
It's first four known printed editions incidentally all pre-date Columbus first voyage, being published in 1472, 1477, 1482 and 1491 in Foligno, Venice and Florence, and adding up to hundreds of copies.
We also know that Columbus had read Claudius Ptomely's work, as well as Pierre d'Ailly's Imago Mundi which depicts the world as a globe. For some books we know this specifically because he owned printed editions and scribbled extensively in the margins.
These books were circulated in some numbers: Columbus did not have to dig up obscure hand written copies or somehow otherwise come across old, forgotten knowledge that nobody else had. He grow up at a time when Italian publishers were churning out rapidly increasing numbers of copies of classical texts all over the place, and works by authors like Ptolemy and d'Ailly (and Dante) who explicitly described the world as round were printed in multiple editions and sought after.
We also known that Ptolemy's work had been sought after in Europe for hundreds of years before that, with adaptations of his Geography, which reflects a round earth, regularly being made and amended and updated with additional information. The first printed edition of his Geography to include engraved maps was made in Bologna in 1477.