Unless you are a pilgrim you might want to spend only a day or two in Lourdes although, apart from the Grotto and Sanctuaries, there are plenty of other places to do day trips to from Lourdes, such as Pau, Tarbes, bus to the Cirque de Garvanie and do a little hiking, take the funicular up the Pic de Jer and walk down.
Avignon deserves more than one day. Spend at least two days there and see Avignon on one day, and the next day cross the river and look around Villeneuve-lez-Avignon which has two old monasteries, the Fort St Andre castle, and the Philipe le Bel tower, among other medieval sites.
There are several little towns called St Saturnin in France, and I'm not sure which one you're referring to, but it might be done as a day trip from the nearest city.
Spend a bit more time in the Loire chateaux area, perhaps stay in Tours and the do day trips to the various chateaux.
Versailles can be a day trip from Paris, and you have time there to include day trips to Chartres and Fontainebleau. Actually you seem to have 17 days in Paris which, surprisingly, might be a little too much. So you could easily add a day or two to the other places you want to visit.
Forget Saint-Etienne, there is nothing much to see there, it's a very industrial place. As you are planning going there from Lyon, could I suggest you take the same train line and change trains at St-Etienne for Le-Puy-en-Velay and stay there for a couple of nights. This is a small town, one of the traditional French starting points for the walking pilgrimage to Santiago de Compostela in Spain. The town is built in an old volcanic area and two of the highlights are a statue and a little church both built on top of volcanic peaks. The Cathedral is also quite different to most you see in France.
Another city you might like is Dijon which has some beautiful medieval half-timbered houses, so you could do Lyon, Le-Puy-en-Velay, Dijon (via Lyon) then Strasbourg.