Spain could rely on American Silver to finance its expansionism, a "blessing" which was not available to France at the same time whose economy was inferior compared to Britain and the Netherlands at that time. I don't mean a quantitative inferiority, but rather a qualitative one, since there was no real banking system in France at that time and agricultural productivity was lagging behind.
True, but English productivity didn't catch up to the Dutch until the 18th century. Why can't we focus on getting the French to catch up earlier?
You're correct France is divided. But even in OTL, it was a near run thing. A France with the Low Countries added would be much more powerful than OTL, especially once industrialization begins.