I'd advise you to read the answer to https://www.reddit.com/r/AskHistorians/comments/235w3l/why_are_the_european_dark_ages_considered_a/ (u/Ambarenya) . Overall the dark ages might not had been as dark as some people think and the ancients Greeks weren’t necessarily as ‘enlightened’ as we make them out to be.

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Also I think it's important to point to out that there is relatively little geographic overlap between the areas settled by the Greek speaking people during 2-3rd centuries BC. and what we tend to understand as medieval Europe. While the literacy rates in 1000 AD France, Germany, Great Britain, Ireland and Spain were relatively low compared to the urban centers of ancient Greece and Rome that's not really relevant since the people who lived there during the ‘Hellenistic period’ (besides some exceptions like the Tartessians and Greek or Phoenician colonies) did not even have a written language.

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If we focus exclusively urban areas, according to some estimates up to 20% of the people living in towns in 12th Britain were able to read which is not that much lower than the lower range estimates for ancient Roman/Greek urban populations. And it was probably quite a bit higher in some cities in medieval Italy.