Place 30 to 40 dried corn husks in a very large bowl or sink and cover with boiling or very hot water. Weigh them down with a heavy plate and soak for 30 to 45 minutes until fully soft and pliable, then drain and cover with a damp towel.
Place pork shoulder chunks, quartered onion, 4 garlic cloves, 2 bay leaves, 1 tsp salt, and 6 cups water in a large stockpot. Bring to a boil, reduce heat to medium-low, cover, and simmer for 1 hour 30 minutes until the pork is very tender.
Remove the pork and let it cool for 10 to 15 minutes, then shred finely with two forks. Strain the broth through a fine mesh strainer, measure out 2.5 to 3 cups, and set aside to cool slightly.
Simmer the ancho and guajillo chiles in 4 cups water for 10 to 12 minutes until softened. Transfer chiles to a blender with 4 garlic cloves, 1 cup soaking water, cumin, oregano, and salt, then blend until smooth and strain through a fine mesh strainer.
Heat a splash of oil in a large skillet over medium heat, pour in the strained chile sauce, and cook for 3 to 5 minutes until fragrant and slightly darkened. Add the shredded pork, stir well to coat, and cook for 5 to 8 more minutes until the mixture is thick. Set aside to cool.
In a stand mixer with the paddle attachment, beat the softened lard on medium-high speed for 3 to 5 minutes until light, fluffy, and pale. This step is essential for achieving a light, airy masa.
Whisk together masa harina, baking powder, salt, and cumin in a bowl. With the mixer on low, add the masa harina mixture and warm pork broth to the whipped lard in three alternating additions, then beat on medium for 3 to 4 minutes until smooth. Test: a small ball of masa should float in cold water.
Lay a soaked corn husk flat with the wide end facing you. Spread 2 to 3 tablespoons of masa into a 4x3-inch rectangle, leaving a 1 to 1.5-inch border on the sides and top and a 2-inch border at the bottom. Spoon 1 to 1.5 tablespoons of filling down the center, then fold the long sides over to enclose the filling and fold up the narrow bottom end to seal. Stand upright on the tray with the open end facing up.
Fill the steamer pot with 2 to 3 inches of water, arrange tamales upright in the steamer basket with open ends up, and bring to a boil. Reduce heat to medium, cover tightly, and steam for 1 hour 15 minutes to 1 hour 30 minutes, replenishing water as needed every 30 minutes.
Remove one tamale with tongs, let it rest for 3 minutes, and unwrap to test — the masa should pull away from the husk cleanly. If still wet or sticky, re-cover and steam for another 15 minutes. Let the finished tamales rest in the covered pot for 10 to 15 minutes before serving.