Interesting. I just like learning about how well people classified into similar language families can understand each other, seeing as Dutch is considered a "Low Franconian" language. There's a lot more to being able to understand each other than a family tree though of course.
So the Franconian family is often argued as a group, but many of the languages are very distinct and closer to other languages, Rhenish is closer to Upper Saxon while East Franconian is pretty close to Bavarian.
This also comes from the fact that the Franks already had two major groupings from the beginning, the Salian Franks and Ripuarian Franks. With the Salian Franks being the direct ancestor of the Dutch speakers. While the Ripuarian Franks are the ancestors of the Rhenish Franconian people.
Then there is also the High German Consonant Shift and other things.
We know a lot thanks through loanwords in French that came from Old Frankish and are very similar to Dutch.
But yeah, the Belgae, the Franks, etc. all put their mark on the regions they lived in and the Low Countries in general are closer related to groups next to them than groups in their own countries. Brabant for example or Limburg, both split between the borders of Belgium and the Netherlands but sharing much in common with the ones across the border. But Hollanders and Flemish were ofcourse different and Holland kinda would come to dominate the North.