Prepare the seafood: Peel the shrimp, reserving the shells and heads. Steam the mussels with water and lemon until they open, discard any that don’t open, and reserve the cooking liquid through a fine sieve.
Heat a paella pan or large shallow skillet over medium heat and add a splash of extra-virgin olive oil. Add the reserved shrimp shells and heads and sauté until fragrant and lightly browned, about 2–3 minutes. Discard the shells and heads.
In the flavored oil, add the finely chopped onion and red bell pepper. Cook over low heat, stirring occasionally, until softened, 8–10 minutes.
Add the minced garlic and cook for 1 minute more, taking care not to let it brown.
Stir in the grated tomatoes (or tomato puree) and cook until the mixture thickens, 5–7 minutes.
Sprinkle in the sweet smoked paprika and quickly stir for a few seconds to prevent it from burning.
Add the squid rings and cook until they turn opaque, about 5 minutes.
Stir in the bomba rice and toast the grains, stirring constantly, 2–3 minutes so each grain is coated.
Pour in the hot fish stock and reserved mussel liquid. Add the saffron threads dissolved in a little warm stock, then spread the rice evenly in the pan. Season with salt to taste.
Increase the heat to medium-high and simmer without stirring for 10 minutes to form the socarrat (crust) at the bottom.
Reduce the heat to medium-low. Tuck the peeled shrimp and opened mussels into the rice, then cook until the liquid is absorbed and the rice is tender, 8–10 minutes.
For a crispy socarrat, turn the heat up to high for the last minute until you hear a gentle crackling, then remove from heat.
Cover the pan with a clean kitchen towel or foil and let the paella rest for 5–10 minutes off the heat.
Serve the paella directly from the pan with lemon wedges on the side.