Surprisingly Satisfying Cauliflower Chowder

Name: Surprisingly Satisfying Cauliflower Chowder
Ingredients: 5 Tbsp. unsalted butter, divided; 1 large onion, finely chopped; 4 celery stalks, thinly sliced; 6 garlic cloves, finely chopped; 2 tsp. chopped thyme; 2½ tsp. Diamond Crystal or 1½ tsp. Morton kosher salt, plus more; ¼ cup all-purpose flour; 1 small head of cauliflower (about 1 lb.), trimmed, cut into small florets; 8 baby Yukon Gold potatoes (about 10 oz.), scrubbed, each cut into eighths; 1½ cups heavy cream; 2 cups oyster crackers; 2 tsp. Old Bay seasoning; 2 Tbsp. white or yellow miso; Freshly ground pepper; Finely chopped chives or scallions and lemon wedges (for serving)