Part 1!

I understand what you mean, but I wouldn't be too puzzled by the blank space in the sources about this region. It's just the nature of studying history that the vast majority of what we want to know is forever obscured by time.

Before I dive in, I would like to stress in the interest of full disclosure that I studied Germanic peoples and languages in the Early Middle Ages. As such I am not a specialist in this area, but I am a Slavic person with a keen interest in the Early Medieval Slavic history and I do read up on scholarship in the area and I will leave a reading list at the end to give you more resources to look into (in Part 2). You can always contact me privately, if you have questions.

It is very difficult to find sources in English about this, probably because of the Iron Curtain making research for Western scholars inaccessible in Slavic countries, plus the fact that most historians only bother to learn Latin and vernacular languages are sidelined. I would hazard a guess and say that studying the Frankish empire is therefore somewhat limited, since the Eastern Frontier is often forgotten and thus deemed irrelevant.

So why the blank space in the historical record before the 6th century? The traditionally accepted narrative here is that the 400s-600s AD are an era called The Migration Period, or Stehovani Narodu in Czech. Because of this, the original homeland of the Slavs in general is believed to be between the rivers Vistula and Dnieper, before Slavs moved to the west.

We also have to keep in mind that the map of Europe looked quite different compared to now. There were Celtic peoples, Avars, Slavs, Germanic peoples and other nomadic folk living in the non-Greek and non-Roman parts of Europe and that means that life outside of this Classical world was not that well documented and whatever is documented by the Romans or Greeks is generally written from their own bias (easily summarised as "We're the best, because we have the rule of law and they're savages, because they don't have laws", or in Tacitus' case, he wrote from the perspective "The Rome is morally bankrupt and our barbarian neighbours are simpler and more honest, more virtuous. We should see them as an example for Rome.")

In the Roman or very early medieval sources, Slavs are usually referred to as Sclavenes, Venethi, and Antes, and especially Procopius writes more about them. In the Early Middle Ages, Fredegar's Chronicle calls Slavs "Venethi", while an Anglo-Saxon poem Widsith, Gregory of Tours and Paul the Deacon refer to Slavs in Pomerania as "Wends" (Winden/Wenden). The accounts of their religious practices and beliefs are difficult to take seriously, as these authors, especially Christian authors would have no interest in preserving a record of their pagan beliefs and rituals, and if they did, it was to portray them with pagan stereotypes of the time. The theory with Suebi being Slavs comes from Jacob Grimm, but it's largely dismissed. Suebi are the people from the Swabia region in today's Germany.

Now onto the Migration Period. The way this is taught at schools in Eastern Europe, or even more broadly than that, is that due to some pressure from the east, whole cultures just got up and moved towards Western Europe. First it was the Celts, then the Germanic peoples, then the Avars, then the Slavs etc etc. And the way this gets oversimplified is that the whole culture, every single individual would have left with their clan/tribe and go west. This is a very flawed way of looking at things and if you look at the genetic map of Europe, no nation today is genetically "pure" in any way.

What usually happens is that while some people stay, some people do indeed leave and settle elsewhere. And every time that happens, cultures blend over time. Before the 9th century, Slavs had had an uneasy relationship with Avars, a normadic people from Asian steppes, often being part of Avar armies when attacking the Frankish Empire. Avar power before the 9th century stretched from what is now Pannonia, or Hungary to the Black Sea and it is now generally accepted that as time went on, Slavic and Avar cultures co-existed, but Slavs eventually are recorded as helping Charlemagne (Karel Veliky) to destroy the Avar Khaganate. Please note, this doesn't mean the Avars stopped existing, only that they stopped being the "top dog" in the region, the ethnic group didn't go away, but its population assymilated into the Franks and Slavs and Magyars over time.

Afterwards, the probably still pagan Slavs became part of the Frankish Empire. The terrotorial bulk of the Frankish Empire was in modern-day France and the western half of modern-day Germany. Charlemagne managed to subdue the Germanic tribes like the Saxons under his rule, but the rulers after him had a different policy towards the Slavs than just fighting them. Incorporating Slavs into the Frankish realm meant that they would convert to Christianity, which granted them new status in the increasingly Christian world, while the Church priests became sort of like inside agents for the Franks in the Slavic territory (the taxes and tribute from Slavic lands were also quite nice). And the other big reason was that Slavic tribes could be used to defend the eastern borders of the Empire.

But the picture is much more complicated. The Slavic leaders in Central Europe that we know from history, Pribina, Kocel, Mojmir, Rastislav, Svatopluk were the Slavic nobility that saw which way the wind was blowing. They were fine with it at first, but all of them ended up, to a lesser or larger extent rebelling against the Franks.