In a large pot, bring 8 cups water, dried mushrooms, garlic, ginger, lemongrass, lime peel, and lime leaves, if using, to a boil. Reduce heat and simmer for 30 minutes, until fragrant. Strain broth, reserving dried mushrooms and discarding other aromatics, and transfer to a large bowl or container. Season broth with salt and pepper, then taste and add more salt as needed. Set aside. Rinse dried mushrooms, finely chop, and set aside. Wipe out pot.
Using the crude shredding blade, spiralize the sweet potato into noodles. Return large pot to the stove, add 1 tablespoon vegetable oil, and heat over medium-high. Add sweet potato noodles and 1 teaspoon salt and sauté, stirring to coat in oil, until cooked through but still firm, 5 to 7 minutes. Transfer to a large plate or a rimmed baking sheet to cool.
Wipe out pot, then add 1 tablespoon oil over high heat. When the oil is very hot, add the mixed mushrooms and cook, without stirring, for about 3 minutes. Then, stir the mushrooms and sauté until brown all over, 4 to 5 minutes more. Season with 1 teaspoon salt and cook for 30 seconds more. Transfer to large plate or rimmed baking sheet. You can cool the mushrooms on the same surface as the sweet potato noodles, but separate them to prevent overcooking.
Reduce heat to medium-high and add remaining tablespoon oil. Add the white and light green parts of scallion, half the sliced chilis, and chopped dried mushrooms (reserved from the broth). Sauté until the vegetables are soft, 1 to 2 minutes. Add mushroom broth and 4 cups water, increase heat, and bring to a boil. Taste and adjust salt as needed.
Reduce heat to a simmer, add kale, and cook for 10 minutes, until tender. Add the sweet potatoes, sautéed mushrooms, and fish sauce, if using. Taste and adjust salt as needed. When ready to serve, garnish with cilantro, remaining sliced chili, and scallion dark greens. Serve with lime wedges for squeezing on top.