Hi Deflated;
Welcome to the forum.
Did you get that brine recipe from any source? The reason I ask is that it really can not be called a brine. A basic flavoring brine will have about 1 cup of salt (some go as low as half cup), and 1 cup of sugar per gallon of water. Adding 8 teaspoons of instacure #1 into a gallon of water is not going to do much. For a gallon of water you will need at least 3 ounces of instacure; which is roughly 1/3 cup (I am at a different computer so I don't have all my information available for the exact amount). Adding about 1/3 cup of instacure #1, you can reduce your salt by the same amount.
As for the texture, from what you described they may have been under cooked, though I only take my turkey legs to 165°F. Is you instant read thermometer accurate, or maybe you only tested only one leg, and the others were not done yet. When I have many pieced of meat on the same tray I test 3 or 4 different piece on the same tray, to ensure all meat is cooked to the internal temperature I want.
It does sound as if you have plenty of turkey for turkey soup, and casseroles; which is not a bad thing.