France is OP. Do not nerf.
The problem with France that makes it OP, in my experience, is NOT the luck bonus, and my suspicion is that it is because all their big neighbors are losing that bonus too. With luck off you regularly see Ottomans get curbed by neighbors and even byzantine pop back up, you see Muscovy fail and sometimes Novgorod piles into it, you often never see Sweden... but I rarely see France lose with luck on OR off.
My issue with France is not morale. Please keep elan; their uber armies are fine. My problem is generally manpower. I have seen France get ganged and walk away fine; they may have to drop territory, but they are back in the game faster than anyone else. Austria always sucks with manpower and Castile still rarely keeps up, while France gets a boost plus its territories and more if luck's on. If England, Austria, Castile, and Burgundy all attack and win, France will STILL have its army back to 100% first (excepting an unused English one). In the meantime, if it lost a lot than the truce is gonna be long, and Brittany or Savoy etc. are gonna lose their land in the meantime. If France gets crushed as a lucky nation it's because they foolishly gave things away in multiple separate peace deals - I have NEVER seen them fail to outperform pre-napoleon history, ever, unless this happens or I do something to them myself. In my most recent game (byz) I saw them SP repeatedly for war reparations and dumb crap, so my suspicion is this is going to be normal now (AI loves them some war reparations) so France is probably less likely to get carved up than it ever was.
My solution is NOT to nerf France. They should be strong, OP even, and that's a good thing. This game is set to end around the time of Napoleon anyway so don't change the BBB being the final boss. Instead, I would like to see some historical rivalries dropped on them extremely hard. I'd say rivalries with England (of course) and Austria (of course), and with Burgundy and Brandenburg/Prussia. Some kind of malus from the entire HRE would be pretty solid. I don't want France to have it's strength touched, and I rather like how well it can recover from a loss, I just want to see it fail at having friends. Make them ally the Ottomans (which is historical) because nobody likes them back home.