Add all spices to a small bowl and stir together until combined. Rub the entire mixture into the pork. Cover the pork and refrigerate for an hour.
Cut yellow onion into wedges. Place at the bottom of the slow cooker. Remove pork from refrigerator and add to slow cooker on top of the onions. Open can of Dr. Pepper and pour over the pork. Cover slow cooker and cook on high for 7 hours (9 hours on low) or until the pork false apart when you gently pulll it.
Remove pork from slow cooker and discard the liquid. Place pork in bowl of stand mixer fitted with a paddle attachment. Turn on low to shred the pork. Turn off once completely shredded. Taste pork and add salt or pepper, if needed.
For the avocado salsa:
Add all ingredients to a blender. Place lid on top and blend on high until smooth, about 30 seconds.
For the tacos:
Place a damp towel on a plate. Stack flour tortillas on top of damp towel and wrap completely with the towel. Microwave for 30 seconds.
Remove from microwave and fill one tortilla with ¼ cup of pulled pork, 2 tablespoons of salsa, and chopped onion. Repeat with the rest of the tortillas.