There is so much research into Roman history today that it seems like we have a pretty detailed understanding of the goings on during the fall of the republic. It makes me wonder though how much of that knowledge was retained over time and how much was lost and gained again. Did Romans during the 300s know more or less about their history than we do?
They would certainly have access to plenty of works that have been lost or partially lost to us.
For example I am reading right now the history of Rome written in greek by Cassius Dio. It did amount to 80 volumes from the origins up until 229 (he died in 235).
Keep in mind that Cassius Dio was a senator, was sent as governor to at least two different provinces and was twice nominated as consul. So he did have access to plenty of public documents that we do no longer have, and was also both an eyewitness and a protagonist of the political scene covered in the last volumes of his work.
Unfortunately many volumes of this work have been lost, or exist only in fragments, some of which are quotations provided in books of later authors.
We do have the complete volumes only from 36 to 60 (but with some gaps in volumes 55 to 60), that cover the period from 65 BC to 12 BC. We do have a synopsis of the earlier volumes, and fragments from various sources of the later ones.
All in all, there were lots of information in the lost works that would have been available to a reader in 300 AD, but that we do no longer have.