Toast ancho chile pieces in an ungreased skillet over medium heat, pressing firmly until aromatic, then flip and repeat. Soak in hot water for 20-30 minutes until pliable, then drain, reserving the soaking liquid.

Roast unpeeled garlic cloves in a dry skillet over medium heat, turning occasionally until soft and charred. Cool, peel, and add to a blender with chiles, cinnamon, pepper, cumin, oregano, vinegar, and 2 teaspoons salt. Blend until smooth, adding soaking liquid as needed.

Add achiote paste, agave, oil, and one piece of pineapple to the blender. Blend until smooth and ketchup-like. Coat both sides of the pork with marinade, cover, and refrigerate for several hours or overnight.

Heat a large cast iron skillet over medium-high heat. Oil and sear the remaining pineapple pieces until caramelized, then chop and set aside. Cook the marinated pork in batches in a single layer, primarily on one side, until charred. Chop into small pieces.

Combine meat and chopped pineapple on a baking sheet and keep warm. Mix chopped onion and cilantro together. Assemble tacos on warm corn tortillas with meat, pineapple, and the onion-cilantro mixture.