Boil potatoes until tender. Drain and rinse under cold water and set aside in a large bowl. Add cucumber, dill, garlic, lemon zest, and salt to a food processor. Blend until well combined. Drain off any excess water. Add to a small bowl. Stir in kombucha. Add mayo and mix until creamy and well blended. Add mixture to potatoes and gently toss until coated. Add scallions and stir gently. Garnish with more freshly chopped dill.