Why did my shrimp bisque turn out so broth like?
I believe the answer to the question lies in the last paragraph of your question. If you had close to 4 cups of broth, 1 1/2 cups of half-and-half, plus onion purée (which would be mostly water); you're pushing 6 cups of liquid, not including any wine or brandy that you may have used. For that much liquid you would need close to a quarter of a cup of flour, maybe more, to achieve a thick soup if roux is your primary thickener.

Many bisque recipes are thickened much with a purée of the protein. Did you purée the shrimp or some portion of it? 

Look at this highly rated recipe from Ina Garten, she uses 4 3/4 cups of broth (in the final soup), 2 cups of half-and-half and a roux made with 1/4 cup (4 tbs flour). She also purées all of the shrimp, which causes a thicker soup than chunks of shrimp would, and adds tomato paste which would further thicken the soup.

For what it is worth, it is the flour in the roux that thickens liquid (an extra tablespoon of butter is not your culprit), and the liquid and roux must be brought together to a simmer for the liquid to properly thicken. In the case of Ina's recipe, she fully thickened the half-and-half before adding the stock and shrimp purée, that's how she gets away with the final line of hot, not boiling. I would be inclined to add the stock and the rest of the ingredients except the shrimp just after thickening the half-and-half and bring that to hard simmer before adding the shrimp. That would give me an opportunity to taste and gauge the thickness the soup before adding the expensive ingredient (shrimp) last.