Preheat oven to 350°F and position rack center; lightly grease a 9×5-inch loaf pan and line with parchment if desired; spread pecan halves on a rimmed baking sheet and toast in the oven 6–8 minutes until fragrant, then cool and chop, reserving 1 tablespoon for the top.

Dice peaches into 1/2-inch pieces and toss them with 1 tablespoon all-purpose flour in a small bowl to absorb excess juice; set aside.

In a large bowl whisk 2 cups flour, baking powder, baking soda, salt, and cinnamon until combined.

In a separate bowl whisk eggs with brown sugar until smooth, then whisk in oil, melted butter, buttermilk, and vanilla extract.

Pour wet ingredients into dry and stir just until combined; the batter should be thicker than pancake batter but still pourable. Fold in the floured peaches and 2/3 cup chopped toasted pecans, reserving the remaining teaspoon-sized amount for sprinkling on top.

Transfer batter to the prepared loaf pan, smooth the top, sprinkle with reserved pecans and optional coarse sugar or streusel, and bake at 350°F for 55–65 minutes, tenting with foil if the top browns too quickly; a toothpick should come out with moist crumbs but not raw batter.

Cool the pan on a rack for 15 minutes, then remove the loaf to the rack to cool completely before slicing to allow the crumb to set.