Do you speak one of the languages of the countries you listed? If so, I would give preference to that country.
I've done many tours in Europe but have not done one in Spain. My impression of it is lots of arid countryside, which doesn't appeal to me.
As was said above, France has a varied geography and you can decide what kind of terrain you want to traverse. Eastern France has many gorges that are truly fabulous to ride. There is also much history, art, architecture and beaches. Not to mention, WWI and II battlefields and invasion beaches. As long as you're not a vegetarian, the food is good. If you want to camp, many French towns how municipal campgrounds that are inexpensive and well maintained.In 2008, I did month-long tour of France. Here is
the detailed journal of that ride.
Personally, I prefer Italy. One reason is that I speak a bit of Italian. In addition, Northern Italy has a beautiful and varied terrain that includes lakes, rivers, valleys, and mountains. All of which I love to ride through. The food in Italy is fabulous, if you like Italian cuisine. Italians have much respect for bike riders and there are a number of bike paths throughout the country. Like France, there is much history, art, architecture and beaches. Also, Italians are a gregarious people who like to engage strangers. I've done several rides in Italy:
Three weeks in Northern Italy,
from Naples down to Sicily,
from Bolzano and the Dolomites up into Austria, and
from Florence, Italy up into France and Switzerland. A great information site for biking in Italy is
https://italy-cycling-guide.info/
Another option is to ride through several European countries. With 6 weeks, you could start in Amsterdam, ride to Passau, jump on the Danube bike path and ride all the way to Vienna or even Budapest. I greatly enjoyed my ride
from Amsterdam to Passau.
Europe is a fabulous place to ride. With a bit of planning, you should have no problem finding exactly what you are looking for.