Yes, well, my suggested timeline eliminates Constantine I. No Constantine, no Constantinople. No split of the empire.
Then you'd have the empire run into rebels at the ends... Britain was constantly in turmoil in the West, Jews in various levels of civil unrest to outright rebellion for hundreds of years.... raiders from literally all sides, and the ever increasing border making it harder to defend what's already conquered.
And when Egypt rebels, or worse, successfully revolts, the Papyrus shuts off, and Roman literacy falls, again, dark age. With or without Christianity, the ends of the empire cannot hold without regionalizing control... and that invariably leads to the dark age in the West. It may shift it a century either side, but paper, dating to about 200 AD in China, wouldn't be a thing in the west until the second millenium begins... And Parchment is physically harder to make, and requires a lot of animals. One per 8 pages is the quote from the recent Nova episode.
Egyptian independence is the most important driver for the loss of knowledge. The Vandals aren't the cause, but just the final nail of the coffin of Latin Civilization. The switch to parchment does usher in the book instead of the scroll... but going from a soldier's day's pay for a short story to a month for his general...
Without cheap writing material, widespread literacy begins to wain. Roman waxed slates were apparently not widespread, despite their being used in instruction as cheaper than paper... but Romans put anything important on papyrus scrolls. Perhaps drafted on wax on a board. To clear them, leave them on the hearth, laid flat, perhaps even iron them.
