Preheat the oven to 350°F (175°C). Line a loaf pan with parchment paper.
Make the cinnamon swirl: in a small bowl combine 1/2 cup sugar-free granulated sweetener and 1 tbsp ground cinnamon; set aside, reserving 1–2 tbsp for the top.
Whisk together the wet ingredients: in a large bowl beat 3 large eggs, 1 1/4 cups full-fat coconut milk, and 1 tsp vanilla (if using) until smooth. If coconut milk is separated, warm briefly and stir until creamy.
In another bowl combine 2 cups almond flour, 3 tbsp coconut flour, 2/3 cup sugar-free granulated sweetener, 1 1/2 tsp baking powder, and 1/2 tsp salt; whisk to combine.
Fold the dry ingredients into the wet ingredients and mix until a thick batter forms.
Transfer half the batter to the prepared loaf pan and spread into an even layer.
Sprinkle most of the cinnamon-sweetener mixture over the batter, leaving about 1–2 tbsp for the top.
Spread the remaining batter over the cinnamon layer. Sprinkle the remaining cinnamon mixture on top.
Drag a butter knife in a long S-shaped motion down the length of the pan to create a swirl, then gently shake the pan to settle the batter.
Bake on the center rack for 50–60 minutes, or until the top is golden and a skewer inserted into the center comes out clean.
Allow the loaf to cool in the pan at least 20 minutes before removing and slicing; cooler bread slices more cleanly.
(Optional) For the glaze: stir 1 cup sugar-free powdered sugar with 2–3 tbsp water until desired consistency, then drizzle over cooled bread.