Prepare an ice bath in a large bowl.
In a small saucepan, heat 4 cups of the vinegar, the sugar, garlic and thyme until steaming, then remove from the heat and steep for 20 minutes.
Fill a stockpot about halfway with water. Add about 2 tablespoons of salt to the water. Add the remaining 4 cups vinegar, then squeeze the lemons into the mixture and add the halves to the pot. Bring the water and vinegar mixture to a boil, then reduce the heat to a low simmer, and add in the shrimp. Poach the shrimp until the flesh is bright pinkish-orange. Drain the shrimp and then shock them in the ice bath.
In a spice grinder, pulse the peppercorns, chile flakes, fennel seeds and bay leaves until finely ground. In a mixing bowl, combine the aromatics with the steeped vinegar and the olive oil, and blend until well combined. Season the brine with salt.
In a plastic container, add the cooked shrimp to the brine, cover with plastic wrap directly on top of shrimp and refrigerate overnight.
To serve, bring the container to room temperature and, with either gloves or a clean slotted spoon, remove the shrimp and drain.