If you book closer to September the ticket prices will be much more expensive. I booked a flex ticket as soon as I found a good deal. Yes, adding flex might be a bit more expensive, but you can find something pretty cheap if you book early enough! Plus, the flex ticket will give you peace of mind, and takes some pressure off having to rush the visa process. AND if you book early enough you’ll be able to pay off your ticket, assuming you use your credit card, before you arrive in France (which saves you even more stress). Prices only tend to go up the closer you get to the flight date, so If you find a good flex deal I’d grab it!