Sarmatians existed (notably) from 50BC till 100/200AC. Problem:
You're right - it would've been probably before the migration and up to the migration. From what I read, Sarmatia was pretty much done by 200. The local tribes most likely mixed with the Sarmatians quite a bit. It appears that the Sarmatian culture was dominant but did have some characteristics of the already local East German tribes. I think that the tribes would've had a Sarmatian dominant culture though. During the migration period we see those people then get further mixed during the migration period, settling into city-states, and ultimately being united to form a nation in 966.
Yeah there is very very few records about the Veneti, Gepids, etc. Most of what he have is from Roman and Greek politicians and military leaders as well as academics like Ptolemy. The interesting thing though is that their religion is thought to be very similar to Greek and Roman mythology. Like the god of lightning would've been Perun, based on Zeus/Jupiter. We do have some pagan traditions and rituals in Poland though to this day - there is one particular popular one that involves making a female like effigy and drowning her in a river or settling her ablaze.