Combine the sugar, 2 tbs adobo sauce, cumin, paprika, 1 teas salt, and 2 teas black pepper in a bowl. Stir to combine. Using a fork, prick the brisket all over. Slather in the sugar mix and wrap in plastic wrap. Let the flavors mingle in the fridge over night.
When ready to cook, unwrap brisket and place in slow cooker.
In a small heatproof bowl, microwave the onions, tomato paste, garlic, oil, chili powder, and remaining 2 tbs adobo sauce in for 1 minute increments, stirring as needed, until the onion is soft. Transfer to slow cooker. Add the water and cover and cook on high for 5 to 7 hours or on low 9 to 11 hours.
Carefully transfer brisket to a cutting board, tent with foil, and let rest for 20 minutes.
Meanwhile, let the braising liquid settle 5 minutes and skim any fat from the surface. Whisk in the ketchup, vinegar, and liquid smoker. Season with salt and pepper to taste.
Shred the brisket with 2 forks and toss with the remaining sauce.
Make the cheese sauce: In a small sauce pan over medium-low heat, heat the milk until just simmering. Add the Velveeta and Dubliner cheese and stir to melt.
Arrange brisket in taco shells and top with desired toppings, ending with a spoon full of cheese sauce. Sink your teeth in and enjoy.