https://en.wikipedia.org/wiki/Ostsiedlung
Problem is everyone knows this and you make great discovery out of that. Yes, Prussians are heavily Slavic too, east Prussians heavily Baltic. Big surprise xD
No. Volga Germans have nothing to do with eastern Germans, same applies to Danube Swabians and Transylvanian Saxons. Those are west/south German colonists in the east, their ethnogenesis happened in west Germany and it did not include Slavs. They lived in closed off rural communities and did not mix with others, especially not with Slavs of orthodox faith like Russians.
Neither are they very Germanic. They are SOUTH-western Germans, similar to Swabians, Belgians, NE French and heavily Celtic. Only Menonnites are more Germanic because they are from further north, where low German is spoken.
Distance to: Menonnite_Russia
3.31072500 Dutch
3.44651998 German_Northwest
3.65231433 English_Southeast
3.84001302 Dutch_Central
3.92383741 North_German
4.05404736 English_Midlands
4.06294228 English
4.38486032 English_North
4.57801267 Cornish
4.58091694 English_Southwest
4.61123628 Dutch_North
4.65486842 Dutch_South
4.95530019 German_West
5.04638484 Welsh
5.20815706 Orcadian
5.22289192 Flemish
5.26143517 French_Northwest
5.27228603 Scottish_East
5.36900363 Scottish_Northeast
5.50253578 Scottish_North_Highlands
5.56982944 Scottish
5.76000868 Scottish_Gaidhealtachd
5.76035589 Scottish_Southwest
5.81072285 German
5.85683362 Icelandic
Distance to: Menonnite_Russia
3.41801112 English_Southeast
3.46538598 Dutch
3.57466082 Dutch_South
3.87778287 Flemish
3.94067253 German_Northwest
3.98347838 English_Midlands
4.10257236 Dutch_Central
4.10405897 English
4.29306417 German_West
4.63007559 Belgian
4.65716652 English_Southwest
4.72683827 Cornish
4.72958772 English_North
4.76758849 French_Northwest
4.83455272 North_German
4.83597974 Afrikaner
5.22651892 Dutch_North
5.35758341 German
5.57456725 Welsh
5.85293943 Orcadian
5.97292223 Scottish_East
6.06311801 Scottish_Northeast
6.26082263 Scottish_North_Highlands
6.37796206 Scottish
6.48822780 Scottish_Southwest
Distance to: Menonnite_Russia
1.99496867 North_German
3.15053964 German_Northwest
4.38270464 Dutch
4.67571385 Dutch_Central
4.98213810 Dutch_North
5.17239790 German
5.43030386 German_West
5.52345906 English_Southeast
5.91426242 English_Midlands
5.94951259 English
6.02376128 Denmark
6.10449015 Dutch_South
6.11571746 English_North
6.19279420 Icelandic
6.45441709 Norwegian
6.47201669 Sweden_Götaland
6.47607906 Flemish
6.53181445 Norway_South_Central
6.59884838 English_Southwest
6.62998492 Swedish
6.65099992 Cornish
6.72165902 Afrikaner
6.79650646 Orcadian
6.94314050 Welsh
6.95171921 Scottish_East
Target: Menonnite_Russia
Distance: 1.9025% / 1.90248230 | R2P
58.0 German_West
42.0 Norwegian
Target: Menonnite_Russia
Distance: 1.9138% / 1.91382998 | R2P
67.5 Flemish
32.5 Denmark
Target: Menonnite_Russia
Distance: 1.2272% / 1.22719454 | R2P
86.1 Norway_South_Central
13.9 Greek_Istanbul