Season and Cook the Shrimp: In a bowl, combine the shrimp with cumin, cayenne, salt, and black pepper until evenly coated. Heat olive oil in a skillet over medium-high heat, and cook the shrimp in a single layer, working in batches if needed to avoid crowding. Cook for about 1 minute per side, or until the shrimp turn pink and opaque. Remove from the pan, roughly chop the shrimp, and set them aside.
Prepare the Pico de Gallo: In a mixing bowl, combine the diced tomatoes, cucumber, red onion, and cilantro. Add 2 tablespoons of lime juice and the olive oil, then gently mix everything together. Taste and add a little more salt if necessary.
Make the Guacamole: Slice the avocados in half and scoop out the flesh into a small bowl. Add the remaining lime juice and the salt. Mash the avocado with a fork until it’s roughly smooth, leaving some texture if preferred. Adjust seasoning as needed.
Assemble the Tostadas: Spread a generous layer of guacamole onto each tostada. Top this with a handful of shredded lettuce and a portion of the cooked shrimp. Spoon the pico de gallo over the shrimp, then garnish with additional cilantro. Finish with a squeeze of lime juice and a drizzle of hot sauce, if desired.