Use unsweetened pumpkin purée, not pumpkin pie filling. The filling is already sweetened and spiced and will wreck the balance.
Let the batter rest the full 15 minutes. The flour hydrates and the leaveners wake up, which is the difference between fluffy and gummy.
Cool waffles on a wire rack in a single layer. Stacking traps steam and the crust goes soft within a minute.
If the batter runs thin and the waffles spread flat, whisk in 2 to 3 extra tablespoons of flour.
Serve with warm maple syrup, honey butter, toasted pecans, or a cloud of whipped cream.
Freeze cooled waffles flat on a sheet pan, then bag them. Reheat straight from frozen in a toaster.
No buttermilk? Stir 1 tablespoon lemon juice into 1.75 cups whole milk and let it sit 10 minutes.