That's over a much longer time period tbf. In the last 1000 years alone we've had the Swedes, the Danes, the Poles, the Holy Roman Empire, Russians (both in imperial and soviet variants), nazi Germany for 3 years, etc.

If we go back as far as the Younger Dryas nobody knows for sure what happened here because so little is known about the different cultures that lived here. Written history mostly starts with the crusade in the 13th century. Before that there's only mentions of conflicts with Scandinavians. Go back further and we're talking about comb ceramic culture and corded ware culture which apparently come with their own genetic markers due to the fact that the cultures themselves were brought by the movement of Yamnaya people for corded and fuck knows what people for comb - genetically it was a mixture of mostly eastern hunter-gatherer with some western hunter-gatherer and western steppe herder thrown in, but the actual people don't have name. Narva culture and earlier was more western hunter-gatherer genetically. It's a huge mess. But same could be said for most continental people, real genetic isolation only really happened on remote islands.