I can't fault your Slav history,it ties in with what I've read too.But I'm no expert,maybe a true Slav historian could pick holes in your statements.
What I wrote is sum up of all what I ever encountered on internet on topic. Story made out of my conclusions on texts by non-Slavs and Slavs, linguists, historians, genetics, etc.
Anyway, holes in my above statement could be of technical nature. In sense, we DO KNOW that Sarmatians (or as they called themselves- Serbians- in many different local versions of that name) covered entire Europe, Eurasia, Near East and parts of Asia and Northern Africa deep in past. And when I say deep past it mean at least last 3.000-5.000 years and then most probable, considering that Sarmats needed time for geographical positioning, we actually speak of at least period between last two glacial periods (so minimum in last 12-15.000 years). Now, what we don`t know with certainty are only two things.
One question is were Sarmats alone, when they covered entire continent, when we speak of Europe? In my opinion, if I were to take my final conclusion it was them, Sarmats, their direct ancestors in lineage, who gave birth to Whites. So, it was most probable, what was White was Sarmatian. It was so in the beginning, in the morning of White race. Under that name- Sarmats (Serbs), Whites were born and plus, parallel with it went name ROS or RAS. That is another open question. Why we always in all old data about Sarmats have that dualism Sarmatian/Thracians for one and same people. Most recent example of it we see in the name of Serbian medieval state where we have Serbia and Rashka and old data confirms with certainty that when we say Rashka we speak of Thracia (pronounced by Greeks/Romans while original was Raska > Th-Rashka > Thracia).
Still, frankly, we can`t tell with certainty were Sarmats alone (only Whites) in deep past, say about 5.000-10.000 years ago and earlier. But, don`t be confused here. It is rather rhetoric question. If other Whites beside Sarmats ever existed it was so deep in past that no trace of them remained, for some reason and only Sarmats prevailed and remained solid in covering of entire Europe. It is also possible that they were also alone in Eurasia. Romans never mentioned other Whites then Sarmats (yes, Celts were Sarmats, too). Romans were specific and they said that Sarmats were within Roman borders and were outside barbarians. With tose within Roman borders Rome actually fell in symbiotic relationship and it is only reason that Rome even survived so long and stretched its rule so far.
So, that is about Sarmats (ie Slavs). Other today`s known Whites of Europe came from them. We know exact mechanism how that happened and it was not that deep in past. Main reason is Roman impact on Sarmatian Europe that led to deeper segmentation of Sarmats, their cultural (in some cases, genetic, too) mixing with foreigners delivered (settled) by Romans into the Sarmatian realm. Only Slavs (ie Sarmats)nstary in direct cultural and genetic lineage with old Sarmats. Sure, it is quite possible that in some cases, those newly formed ethoses have purer Sarmatian genetics then some today existing Slavs. For example, Irish are pure Sarmats by genetics and Bulgarians have foreign add-on in some extent. Still, in cultural sense Bulgarians preserved their direct cultural connection to their Sarmatian meta-culture, while Irish lost it under pressure of new emerged culture.