Cook the pasta in a large pot of salted boiling water as the label directs. Drain and toss with the milk in a large bowl; set aside.
Meanwhile, cook the bacon in a large skillet over medium-high heat until crisp. Drain on paper towels. Discard all but 3 tablespoons drippings from the pan. Add the tomatoes, thyme and garlic to the pan and toss until warmed through; season with salt and pepper. Crumble the bacon into bite-size pieces; set aside 1/4 cup for garnish. Toss the remaining bacon and the tomato mixture with the pasta.
Mix the mayonnaise, sour cream and 3 tablespoons chives with the pasta until evenly combined. Season with salt and pepper. Add the lettuce; toss again to coat. Garnish with the reserved bacon and the remaining 1 tablespoon chives. Serve at room temperature.