Place rinsed navy beans in a large bowl, cover with water by at least 2 inches, and soak overnight at room temperature. Alternatively, boil for 2 minutes, remove from heat, cover, and let sit 1 hour; drain and rinse thoroughly.
Cook chopped bacon in a large skillet over medium heat for 8–10 minutes until crispy, then transfer to a paper towel-lined plate with a slotted spoon, leaving about 2 tablespoons of drippings in the pan.
Add diced onion to the skillet with the reserved bacon drippings and sauté over medium heat for 5–7 minutes until softened and translucent, scraping up any browned bits for extra flavor.
Whisk together maple syrup, ketchup, molasses, brown sugar, apple cider vinegar, Dijon mustard, smoked paprika, garlic powder, salt, and pepper in a medium bowl until the sugar dissolves and the mixture is smooth.
Lightly spray the slow cooker insert with cooking spray, then add the drained beans, cooked bacon, and sautéed onions. Pour the sauce over the top, add 4 cups of water, and stir gently to combine—the liquid should just cover the beans.
Cover and cook on low for 8–10 hours, stirring once halfway through if possible; check at 8 hours—beans should be tender but not mushy. If too thick, add ¼ cup water; if too thin, remove the lid for the last 30 minutes.
Taste and adjust salt, pepper, or maple syrup as needed, then let the beans rest on the warm setting for 30 minutes for deeper flavor. Serve hot, drizzled with a little extra maple syrup and garnished with fresh parsley if desired.