On the contrary, the Germanic people were heroic fighters, defending German culture and language against the decadent Roman Empire, who wished to conquer and enslave them! Particularly the German hero Arminius, who destroyed three Roman legions and threw the Romans out of Germany forever!

Hitler didn't have to fit this into his racial narrative, that had already been done for him. While the origins of the veneration of Germanic tribes of old goes back to the rediscovery and popularization of the writings of Tacitus and Cassius Dio and Paterculus, it really emerged to the fore with the rise of German national identity in the 19th century. Arminius, very often (wrongly) associated with the German name Herman, was the central hero of this German cultural awakening. A giant statue was erected in his memory (Das Hermannsdenkmal) in 1838-1875. He was the subject of a number of plays and operas as well.

Thus the idea of Arminius as Liberator, and of the noble Germanic tribesman, was already firmly entrenched in the German consciousness. All Hitler and the Nazis had to do was co-opt the myth for their own ends.

Murdoch, Adrian Rome's Greatest Defeat Winkler, Martin Arminius the Liberator: Myth and Ideology

Related question, I seem to recall Hitler expressing annoyance of some sort at the comparatively primitive archeological finds being dug up (under Himmler I think?) If anyone could refresh my memory, or even better expand on that it would be fantastic

I can't comment as to Hitler's knowledge of ancient European history. Hopefully someone else can come along and address that.

I would like to say that I find the question, as phrased, very problematic. Historians aren't very comfortable with using terms like barbarism and civilization. They're inherently biased and normative and have a great deal of baggage behind them. To the extent that barbarian is used, it's usually used in its original meaning, to denote "someone not Greek or Roman."

That aside, the Germanic peoples weren't fighting "against Civilization." That implies that Rome was synonymous with civilization, that by fighting against Rome the Germanic peoples were fighting against civilization itself, and that the groups were in a perpetual state of war. The Germanic peoples were not even close to a singular entity; they were composed of many, many different groups whose only real connection was that they spoke related languages. At various times some of these groups allied with Rome; others stayed neutral; and yes, some waged war with Rome. They had no ideological basis to fight against Rome, and indeed, there is strong evidence that many of the groups which violently immigrated into the Empire in the 4th and 5th centuries came not to destroy Rome and civilization, but because they wanted what Rome had and were willing to fight for the right to enjoy the benefits of Roman life.

But they also had war waged upon them. Let's not forget that Rome was an aggressive, expansionist empire with a tendency to intervene militarily in the affairs of its neighbors. Peter Heather illustrates a common fate of Germanic kings living too close to the Roman border in his book The Fall of the Roman Empire: A New History of Rome and the Barbarians. A king of the Germanic Allemanni referred to as Chnodomarius (probably not his real name) became over powerful during a time of temporary Roman weakness. When the Roman situation stabilized, they crossed the Rhine and utterly defeated Chnodomarius. A few years later, another king attained similar power and was subjected to the same treatment. The Romans wanted weak, pliant client rulers on their borders, and were fully prepared to wreak death and ruin on any that seemed even capable of threatening Rome.