Slice the squid tubes into 1/2 inch rings and pat the rings and tentacles completely dry with paper towels. Place them in a bowl, pour the buttermilk over top, and toss to coat. Let them soak for 15 minutes.
In a separate large bowl, whisk together the rice flour, cornstarch, garlic powder, smoked paprika, salt, black pepper, and cayenne pepper.
Heat about 2 inches of avocado oil in a large heavy pot or Dutch oven over medium-high heat until it reaches 375°F. Line a baking sheet with paper towels and place a wire rack on top.
Working in small batches, lift the squid pieces from the buttermilk, shake off excess, and toss them in the seasoned flour mixture, pressing gently to coat all sides. Shake off any extra flour.
Carefully lower each batch into the hot oil and fry for 1 1/2 to 2 minutes, until golden brown and crispy. Do not overcook or the calamari will become tough.
Transfer the fried calamari to the wire rack immediately. Repeat with the remaining pieces, making sure the oil returns to 375°F between batches.
Arrange the fried calamari on a serving platter, sprinkle with fresh parsley, and serve right away with lemon wedges and marinara sauce for dipping.