In a large bowl, toss cabbage, apples, potatoes, and onions with 3 tablespoons of oil and a generous sprinkling of salt and pepper. Mix remaining 3 tablespoons of olive oil with vinegars, caraway, thyme, and garlic powder. Drizzle a generous 3/4 of the seasoning over the cabbage mixture and toss to coat again. Turn onto a large, rimmed baking sheet and shake pan to distribute evenly. Roast until vegetables are tender and starting to brown, about 20 minutes. Remove pan from oven.