Cool completely before glazing. Glaze on a warm loaf soaks in and disappears instead of setting on top.
Apple choice. Firm, tart apples (Granny Smith, Honeycrisp) hold their shape through the long bake. Chop them small (¼-½", 6-12 mm) so they cook through.
Don't over-swirl. A few knife passes gives distinct cinnamon ribbons; more and it blends into a uniformly brown batter.
Tent if browning fast. A full hour at 350°F can over-darken the top — cover loosely with foil for the last 15-20 minutes if needed.
Weighed, not scooped. Butter, sugars, and flour are given by weight — a scooped cup of flour can run 20% heavy, which is the difference between a moist loaf and a dry one.
Original sugar amount. The source lists "⅔ cup + 3 tbsp" granulated sugar by volume but only uses ⅔ cup in the batter and 2 tbsp on the apples. The extra tablespoon (about 0.45 oz / 12 g) is unaccounted for; sprinkle it over the top before baking if you want extra crunch.