Fill a large stockpot (at least 10–12 quarts) ¾ full with water and pour in the beer. Bring to a rolling boil over high heat.
Add Old Bay seasoning, Cajun seasoning, cayenne, salt, sliced lemons, quartered onion, halved garlic heads, and bay leaves. Stir well and let simmer for 10–15 minutes to develop flavor.
Add baby red potatoes to the boiling broth. Cook for 8 minutes uncovered.
Add corn on the cob pieces and sliced andouille sausage. Cook for 5 minutes.
Add the king crab legs. Cook for 5 minutes.
Add the lobster tails. Cook for 5 minutes.
Add the mussels and/or clams. Cook for 4–5 minutes until shells open. Discard any that remain closed.
Add the jumbo shrimp last. Cook for just 2–3 minutes until pink and curled. Do not overcook.
Reserve 1 cup of the hot boil broth, then drain the remaining broth from the pot.
To make the Cajun garlic butter sauce: Melt butter in a saucepan over medium heat. Add minced garlic and sauté 3–4 minutes until fragrant. Add Cajun seasoning, Old Bay, lemon juice, and reserved broth. Simmer for 2–3 minutes. Stir in chopped parsley.
Transfer all seafood, sausage, corn, and potatoes to a large baking sheet, platter, or newspaper-lined table.
Pour the garlic butter sauce generously over everything. Serve immediately with extra lemon wedges and dipping sauce on the side.