Even Kievan Rus is not an exception to the rule; they were incessantly raided and pillaged by the Cumans, and eventually collapsed due to the Mongol invasions from the east. Kievan Rus existed at the turn of the first millennium AD, when eastern Europe had some semblance of civilization, where as in your proposed period, virtually nothing was known about the Eurasian steppe and communication across such distances of wilderness were impossible.
Gotcha...
So, a strong and friendly government on the steppe providing a buffer. This is an idea I have been playing with, but I'm not really sure quite how to accomplish it in the period in question. I have read that the Siraces were a particularly Hellenized group of Iranians, so maybe they could get more organized, but that's a separate order in and of itself.
Even Kievan Rus is not an exception to the rule; they were incessantly raided and pillaged by the Cumans, and eventually collapsed due to the Mongol invasions from the east. Kievan Rus existed at the turn of the first millennium AD, when eastern Europe had some semblance of civilization, where as in your proposed period, virtually nothing was known about the Eurasian steppe and communication across such distances of wilderness were impossible.
This is true, though to my understanding, the collapse of the Kievan Rus had more to do with internal decline than it did an external conquering force. With that in mind, was the original conquest of the area by Scandinavians not motivated by the Dnieper/Volga trade? I understand that the bulk of the trade goods are further south, but there are some good products to be had further north as well. Amber comes to mind, and I have read (and this may not be entirely correct) that it was one of the primary goods traded from north to south both during the 1st century and onward. It's just that during the 1st century, the Eastern European river systems weren't well enough explored yet by merchants to make use of them for such a purpose. Although, settlement of the Dnieper might also be by way of penal colonies for timber, and possibly for religious refugees as well, say... if Greeks in Anatolia are being pressured to convert to a certain religion while the Greeks up north stay pagan.
I dunno. Both historically and hypothetically, earthwork ramparts and other such defenses can be utilized to great effect. Aligning with people's skilled in archery will be necessary, but it's not as if Hellenic states never scored victories against horse nomads. Especially if this is in the Roman era and the resources of an empire can be turned to this project.
Big problem? This becomes the new floodgate so to speak. Successive waves of nomads will come up against Hellenic settlements first. I can't imagine this project doesn't have a short shelf life as an independent state - although that certainly doesn't preclude settlers surviving for a lot longer.
The Greeks of Crimea have apparently survived into the present, something I wasn't aware of until I was reading about the subject recently. They definitely aren't the majority, but they're still present... albeit the Greek community was moved to a city on the Sea of Azov some time in the 20th century, I believe. In the long term, whatever state arises out of the situation might not speak Greek, but Greek colonization of the area wouldn't be without its long-term ramifications in terms of cultural development. A Hellenized Sarmatian kingdom comes to mind, here. If it can be pulled off, I really like the idea of some kind of a mixed language developing, like Cappadocian Greek, but a mix between Proto-Slavic and Greek with a heavy Sarmatian superstrate.