Some obvious stuff.Stay off A roads (motorways) and N roads (trunk roads)
Some other stuffThe D roads with one or two following numbers are generally excellent, quiet and smooth tarmac. Some of the D roads with 3 or 4 following numbers are a bit like A roads. Choose the latter carefully, though my mapping shows whiuch are essentially A roads and which are not. C roads are a bit like our back roads and can be excellent short cuts or a bit agricultural. So again pick wisely and use a bit of streetview to get an idea. The French are quite good at putting road numbers on their signs even for small roads. French drivers are much more considerate unless you are on a road used for busy holiday traffic.
The French have greenways called Verte (such as Avenue Verte) these are generally exceellent with smooth tarmac. Canals often have smooth tarmac towpaths so well worth a look.
Rural France really can be a bit empty and finding somewhere to eat or drink hard. Places aren't open 24/7. So in planning your route take a look at the places you pass through to see if there are food / drink options there. It is worth knowing where along your route the cafes, bars, supermarkets, bakeries are etc.
I have found cycle.travel pretty good at coming up with an initial route that I can tweak. It didn't exist in 2015 (or at least I wasn't away of it) but of all the online planners out there its probably the one I use the most. Mostly I plot routes in Basecamp on the same OSM custom maps I have on my GPS. I always saved the finished article as track for navigating. No unexpected reroutes then.
My custom Basecamp (and GPS) mapping of France looks like this. My UK, Ireland, Spanish, Austrian, Switzerland maps etc. have exactly the same styling and feature setup.
I show the vertes in green and distinguish between paved and unpaved.