Season pork with salt and pepper on all sides. Heat olive oil in a large skillet over medium-high heat. Once hot, add the pork and brown on all sides, about 5 minutes per side. Remove to a cutting board.
In a small dish, combine the cumin, adobo seasoning, and garlic powder; mix well and rub over all surfaces of pork. Using a sharp paring knife, insert blade into the pork on all sides, inserting slivers of garlic into each hole, repeating with all of the garlic.
Transfer pork to a slow cooker and add chicken broth, chipotles in adobo, and bay leaves. Cook on low for 8-10 hours. Remove to a cutting board and shred meat using 2 forks. Drain all but ½ to ⅔ cup of the liquid in the slow cooker and return meat to the slow cooker; cover and allow to cook an additional 5-10 minutes.
Meanwhile, mash the avocados against the side of a bowl using the back of a fork; add salt and combine well.
To assemble, spread avocado over the surface of each tostada and top with 3 ounces of shredded pork, lettuce, tomato, and salsa, if desired. Serve immediately.