I found Flanders to be quite a useful puppet, but it does not depend on France not surrendering. Usually France never surrenders anyway, but only Vichy France is created while free France continues. Be that as it may, Flanders builds many modern infantry and, IIRC, also has the occasional plan it gives Germany. But that will be unlikely with Skill 3 tech teams since Germany has a Skill 9 with 3 icon match to tackle that doctrine.
Flander's units have good ESE as their capital Brussels is usually 185% infra. However, it does not build brigades and I mostly use them as a Reserve force instead of guarding the Atlantic Wall directly. But once the Wehrmacht goes overseas to conquer America, then Flanders (and preferably Wallonia too) are perfect to man most of the Atlantic Wall by themselves given the lower threat then.
But if Germany player has not gained air dominance I would be cautious about liberating Flanders, especially if the Allies are doing Airport Strikes as Flanders has 3 frontline airports which - if damaged - would repair slower than if Germany repaired them.