For you to use Google Flights effectively, you need to start with some specific dates, even if they are not 100%. You can click on Dates See More to get a date grid that will show dates and price changes for each day. If your dates are not set in stone, you may be able to adjust them to get a better price.
It looks like you want to start your trip in Toulouse and end it in Bilbao. Trying to book all the way through to and back from those locations will probably cost a lot more than making separate reservations for the intra-European flights.
Not knowing your exact dates, it appears that you could save hundreds of dollars at that time of year by flying RT nonstop Chicago -> Madrid or Paris on many dates in late June and early July. The prices showing at the moment are about $1000 or less.
Flying from Detroit increases the RT price to Madrid by $590 - $710 depending on the day with no nonstop options. To Paris the increase is $320 - $490 depending on the day for nonstop flights. That's so much that flying from Chicago, it would still probably be cheaper even if you had to spend the night at ORD.
You can also search for times and prices for separate flights Paris or Madrid -> Toulouse and Bilbao -> Paris or Madrid. For late June and early July I saw very cheap prices ($19 - $36) for Madrid to Toulouse, but they are late in the day. The ones from Paris are 2-3 times as much with more inconvenient times.
The Bilbao -> Madrid or Paris timing is much better, assuming you go the day before your flight back to the States. The prices for Madrid range from $39 - $54 and those to Paris range from $43 - $131.
The train is always an alternative, especially for the arrival day. Madrid to Toulouse is about 7.5 hours. From Paris, it's about 5.5 - 7 hours. The train might be a better and more fun option and get you to Toulouse faster, but it could be more expensive. At least you wouldn't have to wait around the airport all day.
From Bilbao to Madrid by train is about 6 hours. To Paris CDG it's about 13 hours, so I wouldn't even consider that an option unless I wanted to see the countryside.