Combine salt and water with 3 cups water from the 7.5 litres into a saucepan. Heat, stirring continuously until salt and sugar are completely dissolved. Take off heat and cool fully.
Pour salt/sugar mixture into a brining bucket with the remaining water, beers, half the garlic and bay leaves. Remove the giblet bag and neck from bird, fully submerge bird in brine and soak, chilled, for 12-24 hours.
Rinse turkey thoroughly and place in a roasting pan, tie legs if desired.
Mix herbs, remaining garlic and butter together, rub herb butter all over turkey skin.
Cook turkey at 350 degrees F for 1 ½ hours, reduce heat to 325 degrees F and cook until bird reaches 165 degrees F in the deepest part of the meat. Remove from oven and let sit for 20-40 mins at room temperature. Reserve drippings for gravy.