Preheat oven to 400 degrees F (convect/fan). Peel the purple sweet potato and make crosswise sliced around ½ cm thick. Then cut each slice into 4 pieces. Place pieces onto a large baking tray, drizzle with 1 tbsp. avocado oil, and toss. Sprinkle with ¼ tsp. kosher salt and roast for 15 min. Toss, sprinkle with remaining ¼ tsp. kosher salt, and roast another 5 to 10 min until tender and beginning to crisp.
Cook 1 ¾ cups cavatappi pasta in salted water until al dente, according to package directions, then drain and rinse under cool water.
Add all Avocado Dill Dressing ingredients (1/2 large avocado, 2 tbsp. fresh dill, 1 tbsp. dijon mustard, 1 tbsp. agave nectar, 1 tbsp. lemon juice, ¼ tsp. kosher salt, ¼ tsp. black pepper, ¼ tsp. garlic powder, ¼ cup vegan mayo) to a food processor and blend until smooth, scraping down sides as needed.
Add 5 oz. baby spinach to a serving bowl or platter. Carefully cut faces into some of the cucumber slices using a small paring knife, if desired. Top the spinach with cucumbers, purple potatoes, 1/3 cup toasted walnuts, 3 tbsp. dried cranberries, ¼ cup crumbled vegan feta, 3 tbsp. thinly sliced scallions, and ½ cup (packed) thinly sliced red cabbage.
Mix the pasta with just enough dressing to lightly coat it, then top salad with the pasta. Serve salad with remaining dressing.