For the taco filling: In a small bowl, combine the cumin, chili powder, paprika, oregano, and salt.
Place the ground turkey in the slow cooker, breaking it up loosely with your hands or a wooden spoon. Add the beans, top with the spice mixture, and pour the salsa or tomatoes over everything.
Cover and cook on low for 8 hours, until the meat is tender. Using a wooden spoon, break up the ground turkey and stir until everything is well combined.
For the guacamole: In a medium bowl, combine the avocado, tomato, onion or onion powder, garlic powder, lime juice, cilantro, salt, and pepper. Taste and add more salt, lime juice, or cilantro as needed.
For the tacos: Warm the tortillas directly over medium flame on the stove, about 20 seconds each side. Place 2 on each plate.
Assemble each taco by placing a 1/4 cup of the taco filling on each tortilla. Use a slotted spoon to minimize liquid in the taco. Divide guacamole and lettuce among the tacos. Top each with 1 tbls of the cheddar and sour cream/greek yogurt (if using) and serve.