Start by making the corned venison brine. In a saucepan, add 8 cups water, Tender Quick, 2 Tablespoons kosher salt, brown sugar, 2 Tablespoons of the pickling spice, and minced garlic and boil for a few minutes until the Tender Quick, salt, and brown sugar have completely dissolved.
Remove the saucepan from the heat and let corned venison brine cool completely in the fridge. Once it has cooled, place the venison roast in a 2-gallon Ziploc bag and add the corning brine. The brine should cover the meat. Seal and lay flat inside a 9 x 13 baking dish and place in the refrigerator for 4 to 5 days. Flip the venison roast over every day, so that all sides get brined equally.
Bring a large pot of water to a simmer over medium-high heat. Remove corned venison roast from brine and rinse thoroughly under cold water; discard brine. Add corned venison roast to simmering water along with reserved 2 Tablespoons of kosher salt and 2 Tablespoons pickling spice. Partially cover the pot with a lid. Reduce heat to medium so it's simmering (not boiling), and cook until corned venison is fork-tender and easily shreds with the grain, about 3 hours.