A strategy suggestion. We will not build our life around waiting in lines and the popular sites will have very long lines. What I did last year and will do again this year is look for lesser known sites near where we are staying. Last year I went to a high school that had been designed by Guimard the art nouveau architect who designed the attractive metro entrances around Paris. It was very interesting -- gorgeous structure. And no line; no crowd; they were happy to see visitors.
I also went to Citeco, the gorgeous bank of France headquarters in the 17th at Place General Catroux -- you get to tour the stunning old chateau and there is a big museum of economics and money that actually turned out to be rather interesting. they preserve the old bank vaults and such but also display coins throughout history, money presses, and a museum of how the economy works (from the perspective of capitalists of course). Again, very interesting and no lines at all and no crowds. More people than at the high school, but no line.
both of these were in the 17th where we always stay in the fall. We will be looking for similar opportunities this fall. Look for things that are either expensive or closed the rest of the year.