Rub the beef roast all over with olive oil, chopped chipotle peppers, smoked paprika, cumin, chili powder, garlic powder, salt, and pepper.
Optional: In a hot skillet over medium-high heat, sear the roast on all sides until browned, about 2 to 3 minutes per side.
Place the sliced onion and minced garlic in the bottom of the slow cooker.
Set the seasoned roast on top of the onions and garlic. Pour the beef broth and tomato sauce around the meat, avoiding pouring directly on the roast.
Cover and cook on low for 8 to 10 hours or on high for 5 to 6 hours, until beef is very tender and shreds easily.
Remove the roast from the slow cooker and shred the beef using two forks.
Return the shredded beef to the slow cooker and stir gently to coat it in the cooking juices.
Warm tortillas in a dry skillet, wrapped in foil in the oven, or briefly in the microwave until soft.
Assemble tacos by spooning shredded beef into tortillas and topping with cilantro, lime juice, and any optional toppings like diced onion, avocado, lettuce, sour cream, or cheese.
Serve immediately with extra lime wedges and desired toppings on the side.