Modern Germans are largely descendants of settlers from the French-Dutch-Belgian-German borderland and from Southern Germany who re-populated Central, Northern and Eastern Germany during the Medieval "Ostsiedlung". In Ancient Germania and in Early Medieval times after the Migration Period, haplogroup frequencies in what is now Central & Eastern Germany were much different than now.
Even Pagan Saxons were decimated by Charlemagne, and huge immigration of West Frankish settlers to their lands took place.
So Northern Germany is also different than it used to be before the bloody conquest of Saxons by Charlemagne. Probably before Charlemagne Northern Germany was dominated by I1 (like modern Denmark is), and only after Charlemagne by R1b.
Remember, that we already do have an Anglo-Saxon ancient DNA sample from England, and it turned out to be I1.
We also have Roman era Y-DNA from Britain, and among it there were 2 samples of R1b-U106.
So the presence of U106 in what is now England actually pre-dated the Anglo-Saxon conquest.
Anglo-Saxons who came to Britain, were probably mostly I1. Just like Pagan Saxons before Charlemagne.
The conquest of Saxons by the Frankish Empire lasted around four decades (years 772 - 804).
We don't know how many died and how many survived, but sources indicate that losses were very heavy. For example in year 782 Charlemagne massacred 4500 Southern Saxon captives. In 795-798 there was enslavement and forcible population transfers of Northern Saxons. In year 804 Charlemagne expelled over 10,000 Northern Saxon families to Gaul - their descendants speak French today, and are no longer parts of the gene pool of modern Germans. In 798 in the battle of Bornhöved 3000-4000 Saxons were killed. These are just some examples of war crimes and casualties inflicted by Charlemagne and his allies against Pagan Saxons - there were more casualties for sure. About the massacre of 4500 captured Saxon warriors in 782: https://en.wikipedia.org/wiki/Massacre_of_Verden
What is now Northern Germany was largely devastated and depopulated during those wars.
Saxon lands were then largely re-populated by western settlers coming from Western Francia.
It seems that Thuringian genetic distinctiveness surived better until the present-day than Saxon.
Thuringia has the lowest frequency of R1b-U106 and the highest of I2a2 out of all regions of Germany.
This is probably the reason why 23andMe cannot genetically distinguish French people from Germans today (see the link):
The same process (depopulation + repopulation by Western settlers) later took place after crusades against Pagan Slavs.
You can read about this for example in the following book:
Gerald Stone, "Slav Outposts in Central European History: The Wends, Sorbs and Kashubs"