My admittedly cursory understanding of early European history is that celtic Indo-European people migrated from modern-day Turkey to Germany (mixing with Scandinavians (?) to become the Visigoths and Ostrogoths), Gallia (Gauls), Spain (the Celtiberians), and the British aisles (Britons) between 7000-2000 BCE.
Later the Gauls were mostly subjugated by Rome who then shared a border with the Germanic tribes east of the Rhine. I know warlords like Ariovistus made forays into Gaul from Germany but were beaten back. Then in the fourth century AD Gothic and Vandal tribes migrated west, displaced by Slavic migrants and the Huns, pushing into France and the Italian peninsula culminating in the fall of the WRE. Please correct anything overtly wrong here.
I'm really struggling to understand how these tribes became the Medieval kingdoms of 1000 AD + Europe. I know they were sometimes large pseudo-nations organized around powerful tribal warlords which is not too dissimilar from Feudalism. I also know Charles Martel and Charlemagne fought the Moors in the Iberian peninsula, adopted Christianity and founded Francia. How exactly did they go from tribes trading with/borrowing technology from Rome to huge, organized Feudal states? How did the Goths and Vandals form the Holy Roman Empire?
Just to address some misconceptions from the original question.
Austria has not been a kingdom as such. It’s been margraviate in medieval times and then duchy and archduchy later on.
Prussia was a protestant duchy which emerged after the defeat of Teutonic Order and was a dominion of Polish-Lithuanian Commonwealth at least until 30 years war (when the wars with Sweden weakened the Commonwealth influence in the area). We are talking about a Protestant state which emerged in the Renaissance and hadn’t become a kingdom until much later. Before Teutonic Order colonized the area, at the invitation of Polish princes, the area has been inhabited by Baltic people known as Proto-Prussians or Old Prussians - a culture which has been entirely exterminated by the Teutonic Order’s crusade. Regardless, there was no such thing as Prussian kingdom in medieval times and for the most of the medieval era the lands were inhabited by Baltic, not Germanic, people. There were Germanic tribes in the area in the Roman times, but that’s before the migration and I don’t know if there were any traces of Germanic settlements at the time of Teutonic Order’s arrival to the area.
Could you speak more as to why many historians don’t believe that strict definition of feudalism didn’t exist? That would be an interesting rabbit hole.