no Sarmatian descent among the Szlachta of the Wielkopolska region - and there was instead a huge Norman and Germanic
Your crucial error is to think of Sarmatians as of people with identical haplogroup. Its so naive. At their maximum Sarmats lived from British islands, over entire Europe, Eurasia, Near East to the Ind river. Even if you exclude mixing with outsiders it is IMPOSSIBLE Sarmats to have same haplogroup on such a large inter-continental space.
Do you understand that? All Europeans are Sarmatians in origin. All are genetic Sarmatians who once also shared their Sarmatian common cultural and linguistic heritage. But there is the trick. Only Slavs remained Sarmatians in every sense. In genetic, cultural and linguistic sense. Only Balkan, Lusatian and Kashub Serbs, from all Slavs, still using local versions of once original Sarmatian name. Other Whites remained only genetic Sarmatians but abandoned their direct cultural and linguistic connection to ancestors. Indirect cultural/lingual connection exist but not direct.
Or to tell this way. People is one thing and nation is the novelty. See, Slavs are still people. They are as were their ancestors. Non-Slavic Whites aren`t same people anymore. They are now nations. They just originate from Sarmatian people but they are something else now. Not new people because they still have Sarmatian genetics. They are new cultures, a nations that originate from Sarmatian people. Most probably exactly that fact explains their atavistic hate on us who remained Slavs ie Sarmatians. As long as we live, live fact that non-Slavic Whites abandoned their ancestral original Western culture, while we who are still Slavs ie Sarmats did not. We remained traditional, true, original.
Pardon. Name of Shwabi, Swedi, Spaniards (possible even some more national names) are also local versions of Sarmatian name. Possible corrupted with foreign influences but as far as I know also could evolve naturally that way due to distance from cultural core and isolation. Those Sarmat names are still in use. Interestingly, name remained but culture and direct connection to ancestral language are lost. And shockingly, those who using them even don`t understand that they bear name of true native Europeans, their original ancestors.