Reddit mentions of Paul and First-Century Letter Writing: Secretaries, Composition and Collection

This seems to be the position put forward by Richards in Paul and First Century Letter Writing.

He helpfully points out that in the ancient world, you could think about "authorship" as a sliding scale from 1-10.

At 10, you have something that the author personally wrote with their own hand. At 1, you have forged documents, claiming to be from someone but really there is no connection. Maybe at 2 you would have disciples carrying on their teacher's legacy and writing in their name. At 8 you would have verbal oral dictation. At 6 you would have a scribe significantly helping the dictator frame arguments and sentences. At 3 you might have something like a request to a scribe "Send a letter back home from me and tell them what I've been doing". The authority is still from the person even though the author isn't the person.

He supports this view by reviewing first century writing practices in the Roman world, from famous and known Roman officials.

As you've already noted, however, the case against Pauline authorship for the pastorals isn't purely on linguistic grounds. Other arguments range from a more developed church structure being present to the letters being responses to heresies that primarily appear in the second century.

Yeah, books in the ancient world were luxury items that cost the equivalent of hundreds of dollars each; writing materials were not cheap, and each copy of a book required the services of a skilled scribe to write it out by hand. (E. Randolph Richards estimates prices for some of these goods and services in Paul and First-Century Letter Writing.) The evangelists would probably each have wanted a copy of their gospel for themselves, plus a few to send to the churches in Christian centres like Rome, Antioch, and Alexandria in the hope they would disseminate it further. A few chapters of extraneous information about Jesus' teenage years = $$$$

There is also some evidence alluding to the fact that he did this because there were forgeries going around. This could also be why the first chapter of Galatians is dedicated to Paul criticizing them for "so quickly deserting him who called you." He then goes on to talk about why he has authority to preach the gospel. If you want to know more about it, check out this book.