When i played with France i ended up with the Absolute Monarchy rather then a republic, but that only came up when i got a king with high skills. Imo playing as France it's prolly easier to break with the vassals then like me, keeping them and hoping for a diplo-annexation. I went for the last and had to wait for nearly 200 years be4 i annexed the last "french"-vassal(Had Savoy, Lorraine and Milan as Vassals also). Getting into a alliance with Castille is most likely also a good thing, for it will secure the southern borders.