Sure, Ancient Literacy was near 10%, but ancient life was communal. If a Synagogue housed at least one scroll, the whole community owned that scroll. If a few people - or several - had a particular urge to hear those blessed words once again, they could schedule an evening and call in one of their literate townspeople to read for them. Although less than 10% of each city was literate, nearly all the Jews everywhere loved literature.
Thus, every literate man was obliged to his Synagogue. Thus, every Jew could be literarily minded. To put that another way, all Jews were communally literate, if not personally literate, to some degree or another. It's no accident they ALL came to be known as "The People of the Book", instead of "The People with some Book People among Them".
Now, it is these people to whom Jesus grew up with and to whom He eventually began preaching. It was of such a people that Jesus selected his disciples, of whom he appointed twelve to be special envoys of the approaching Kingdom. And of these twelve, who saw Him as their incoming King, we ought to suppose perhaps one or two may have been literate. Perhaps more, if Jesus deliberately selected somewhat capable men.
Probably Levi (Matthew) was at least able to keep basic records, in his work as a Publican, but the four fishermen were almost surely illiterate (Acts 4:13). We've not much to go on beyond that, apart from wild guesses like - Thomas, who doubted, may have been somewhat more educated than others. Yes, that's a joke. We have no idea, except that statistically, we might guess that at least one of those twelve should have been able to read.
And don't forget, Jesus also could read. If he could also write basic words, perhaps Jesus tutored his lone literate(s?) towards basic competency in composition. Or perhaps not. However, Jesus also had others who followed along with the twelve for good stretches of time, especially after Joanna and Susanna began paying for everyone's food. Perhaps Joanna also paid for a transcriptionist. Perhaps not. But I hope you see my point. Honestly, speculation isn't what I'm after here.
Overall, these people provide Jesus with more and more opportunities to take on ONE person who could have taken initiative by their own literacy level to start putting down a few written notes, producing early written sources for the Gospels. Yes, that's hypothetical, but so is treating Oral Tradition as if there were no early writings whatsoever.
It's a simple thought, really. Which is more statistically likely? That there were no writings because soemthing like 90% of the population was illiterate? Or that, among the literate folks who did follow Jesus, at least one of them took some pains to keep a journal?
Among known associates of Jesus, historically speaking, I still think Matthew and Nicodemus are the most likely candidates to have actually done so. A Nicodemus journal in the hands of the beloved disciple could explain quite a lot about John's composition. And if Mark had both Matthew's original notes and/or Matthew's finished Gospel to work from, it could help explain at least some of the textual variances among Matthew, Mark & Luke.
Just a theory, but one I continue to like infinitely more than vague "Oral Tradition" alone, or worse, OT with "Q". Although something like a "little q", I can totally see.