As
@Restless Native mentioned, a couple weeks back I released a mod for Eu4, the France Overhaul mod (link is in my signature). I full well understand the problems you have with France and how misrepresented historically it is in the base game atm.
I'm not going to make extremely in depth suggestions on how to improve France, but for the most densely populated region of Europe at the time France is sorely misrepresented (just look at the size of Languedoc and Nemours). Both France and Russia received minor changes in Mare Nostrum (1.16), but Paradox only decides to go ahead and improve the latter again for 1.22.
Additionally, I think the old vassals should be brought back, but with a unique subject type which instead of being annexed in the traditional way by France using dip power, they are instead inherited by France should the ruler of the subject die without an heir. This better represents the system of feudalism that was prevalent in France towards the early modern period, and also means that the new subjects won't just be eaten in a few decades and be forgotten about.
Also, I have an issue with the Inheritance of Anjou event. From what I've seen the event fires somewhere around the 1460's which is far too early for France to capitalise on the Italian states breaking from the HRE due to Shadow Kingdom, as the claims disappear within 25 years. Therefore France never seems to pose much of a threat to Italy or HRE as they historically were.
TL;DR: Make France more challenging and interesting, with more things to hold them back in the early game (having to handle their subjects) whilst also giving them more of a chance to rival the HRE in the mid- late game if they manage to annex all their subjects.