Decathlon and InterSport are decent enough shops, but I wouldn't want to walk in, spend €700 on a bike and kit, and ride 2300km. There's also a Giant store in Nantes. In an ideal world you'd get something second hand and have it serviced (or buy second hand from a bike shop that has already serviced it). Getting all the luggage fitted and everything set up nicely might involve you taking or buying tools, and a trip to a hardware shop
Nantes seems (according to Google maps) to have a decent selection of independent bike shops with good reviews. A chain bike shop that does servicing could also be good. These places will be able to fit racks, extra bottle cages etc. without difficulty.
Even a big shop doesn't actually hold much stock, so your chances of riding out with something suitable are low. You'll need to plan carefully. One advantage of the big retailers is that you might be able to try a bike in a branch near you, and order it in to a suitably located branch to be built up for you. I'd probably go down that route because I struggle to find bikes to fit, especially second hand, but if you're more of a normal size than me, contacting a shop that sells second hand, and explaining what you need, could be very useful. I would write in French with the aid of Google, but include the same message in English underneath.
I've ridden a fair bit (day rides) in the Loire valley, and there are plenty of little bike shops in case things go wrong in the first few days, or you find you need something extra. These aren't necessarily the places that have much stock of unusual parts.