terrible traffic in almost every way for transport modes from bicycles to trains.
Flanders is at the heart of Europe and has to handle more traffic than the Netherlands. Flanders is working hard on its infrastructure, you should be aware of that seeing how you're very aware of how awful they are. Criticizing anything and everything which is being worked on is absurd and quite idiotic if you'd ask me.
terrible urban planning
The urban planning in Flanders and Belgium in general is a result of it being the first industrialized country in continental Europe. Belgium at one point was the second largest economy in the world. They most certainly had a bigger economy than the Netherlands for quite some time.
language difficulties
That is an issue that needs to be handled by Flanders and Wallonia, the Netherlands plays no role in it.
terrible internet (traffic shaping and usage caps happen in Flanders too).
Yup, so terrible that KPN is begging Proximus for a merger. The latter rejecting said offer of course. Telenet, which mostly serves Flanders alone, is of excellent value, though quite expensive. How this is an usable argument against Flanders joining the Netherlands goes beyond me.
The others (prison pop, immigration&integration, GDP/cap, PPP/cap) I can't really find data on for Flanders alone. Of course some of these are because Belgium as a whole made these decisions, but I still stand by my point that I will repeat and translate below.
The Flemish and Belgians in general are richer than the Dutch. They have less debt, on average own more real estate than the Dutch and have more financial assets.
I don't think of Belgium or Flanders as a third world country in any way and I'm sure there are areas where Belgium and/or Flanders do better than the Netherlands, but I think that the Netherlands has plenty of problems on its own without adding Flanders'.
It seems like you do as you keep repeating this part over and over again. Either way, if this will allow you to sleep at night, no one in Flanders wants to join the Netherlands unfortunately. It is an idea that lives more in the Netherlands than in Flanders itself.