For the bacon: Preheat the oven to 400 degrees F. Line 2 baking sheets with foil. Set a baking rack over the baking sheet.
In a small bowl, whisk together the sugar, ancho, paprika and cayenne. Arrange the bacon strips on the rack and sprinkle the tops with all of the spice mixture. Gently push the spice mixture onto the bacon. Bake, rotating halfway through, until sizzling and lightly browned, 22 to 25 minutes.
Remove the bacon from the sheet pan to a paper towel-line plate and let cool. Cut the bacon in half.
For the mayo: In a food processor, add the garlic, cilantro, parsley and chives and pulse, until the herbs are all chopped. Add the mayonnaise, mustard, lemon zest, salt and freshly ground black pepper and pulse until all the ingredients are well combined. Season to taste. Set aside. (Yield: 1 cup.)
To build the sandwiches: Spread 1 1/2 teaspoons of herb mayo on the cut-sides of the bread. Layer 2 tomato slices, a leaf of lettuce, 4 pieces of bacon and 3 pieces of sliced avocado on the bottom halves of the bread. Add the tops and serve with pickles if using.