Now you are really stretching.
De Excidio Britanniae and The Confessio of Saint Patrick are the ONLY two contemporary sources concerning the 5th/6th century ~ outside of the Arthurian materials in the Welsh Annals.
Only two sources of what? Arthur or the general history? Because there are many documents from the 6th c., so where are the contemporary accounts of your mega-disaster? (I think we can safely dismiss 5th c. sources of what happened in 540 AD, don't you?)
I mentioned De Excidio Britanniae, because it was likely written in the 540s, and so exactly when you claim the event occurred. If that isn't exact enough for you, how about Historia Francorum, by Gregory of Tours, written c. 591 AD? Surely such a disaster happening only 50 years before would be mentioned in a definitive History of the Franks.
Now, why is that? Why are they only two? For one thing, they escaped getting burned during the dire conditions at the middle of the 6th century.
Now, why did the Angles and Saxons arrive? Why did they want to leave a vast continental area devoid of enemies just to go to Britain to fight with the technologically superior Britons? Well, it was real bad back home. In fact, the islands the Angels (Frisians) lived on were unlivable and the rest of Germany where the Saxons lived was getting very nasty ~ crop failures were common.
Still, you will note that BOTH documents were written by guys on islands which were not affected as much by the change in climate that was driving people out of the Continent.
Yes, islands ~ Britain is an Island. Scota is an island (and would one day in the future be called Ireland).
As you recall, I noted that folks on peripheral island areas did much better than people on the continents.
A similar phenomenon happened on the East Coast of Eur-Asia. Koreans departed from the Korean peninsula circa 560 and arrived in Japan where they took over and founded the present dynasty. These people were escaping bad weather, a devastated economy, and going to a place where food was still growing.
So, come up with all those other books published in Germany in the 500s ~
You can branch out into the other Gaelic languages from that point. It's all quite interesting.
Important dates: 1085 Islamic Toledo falls to northern Christians. By 1130 translations of books from the Toledo library filter throughought Europe. By the 1300s the Renaissance is underway.
That's a mere 200 years to take one of the most primitive, decrepit, broken-down, impoverished societies on Earth and begin to turn it toward the stars.
Toledo's great library was one of the most comprehensive repositories of ancient books ever created. Upon conquering Toledo the Christians and Moslems agreed that the library must be preserved, and it was done.