There are Germans from Slavic / protoslavic descent in North-Eastern Germany aside from the small Slavuc speaking minorities. Some location names and family names (Wend, Wenden, etc) are very telling in this regard. I like this direct link to ancient history.

It's an open question whether the Veneti themselves were ethnically Slavic though. We know of the earlier Gallic Veneti, which were a Celtic seafaring tribe that settled multiple locations along the European coastline. The Italian region Veneto and the city Venice can also trace their names to an ancient group called "Veneti", and they seem to have spoken a mix of Italic and Celtic. In contrast, the Slavic language seems to have originated much further east.

https://en.wikipedia.org/wiki/Venetic_language :

A 2012 study has suggested that Venetic was a relatively conservative language significantly similar to Celtic, on the basis of morphology, while it occupied an intermediate position between Celtic and Italic, on the basis of phonology

https://en.wikipedia.org/wiki/History_of_Proto-Slavic#Pre-Sl... :

The most archaic Slavic hydronyms are found here, along the middle Dnieper, Pripet and upper Dniester rivers [..] inherited Common Slavic vocabulary [includes] well-developed terminology for inland bodies of water (lakes, river, swamps) and kinds of forest (deciduous and coniferous), for the trees, plants, animals and birds indigenous to the temperate forest zone, and for the fish native to its waters

The fact that the Slavic language seems to have developed inland but the Veneti mostly settled coastal areas puts a big question mark under the Veneti == Slavs link.

Research on this topic has been very active since the late 2000s (before that scholars on the topic had been very few). It seems that the scale of the migrations are much better understood now (100k-300k people over 3 centuries).

But as you say, the rapid transformation of the societies of the groups that migrated at the time is very hard to comprehend, but also the changes of those where they migrated, in cases where it was more of a fusion than an invasion / displacement.

I think it is a very interesting point to offer to those in Europe minimizing the current migration waves (often for noble reasons of reducing racism and countering far right ideologies). It's no use denying that welcoming tens of millions of people from other cultures over a few decades has and will have very deep social and political effects.