Firstly, I think Dutch are underrepresented in genetic studies so far.
Secondly, being Dutch myself, I think following points may be of interest
- The Hollandic coast was Frisian-speaking, and became only later, in the Middle Ages "Dutch"-speaking, a Frankish dialect, which came from upstream.
- A large part, around the IJssel river, speaks Saxon dialects.
- Lots of land in Holland are only later be made into land and populated from around.
- Limburg seems more German-oriented.
- The duchy of Brabant originally started in Louvain, containing Brussels and Antwerpen, and stretched over to Dutch Brabant. The most important area lay in the south.
- The Roman border lay in Dutch lands.
- people from Zeeland are sometimes stereotypically seen as mainly dark-eyed and dark-haired. Commonly this is ascribed as due to Spanish soldiers, but that is not true. Same "dark type" is seen along the Atlantic coast. People from Brabant and Limburg can also be seen as more "dark".
- In the 16th/17th century, many Flemish reformed people fled to cities, mainly in the province of Holland. This was a significally important event, counting up to 20% of people. Note that Holland was and is quite city-oriented with lots of immigration and migration between cities or from land to city. Migration seemed less amongst the farmers.
Evenmore, there has always been immigration from Germany. I think it is rare to find a Dutch person without German ancestry, even if it is only one guy in the 1600s.
- The Dutch knew a culture of pillarization. No religion was dominant, but in throughout four centuries people tended to marry someone of their own religion (which sometimes of course did not happen).
- Frisians seem on the one hand quite isolated, though do a lot with sea faring, so might have contributed in gene spreading.
- The rivers (Waal) are on the one hand barriers for population movement, but have certainly been crossed by a lot of people.
just some points which may seem unorganised, but I think that on the one hand, Dutch and Belgian people are more "one people" than one might think, though are also characterised by different types of groups within the society.