Heat the oil in a large stock pot over medium heat. Cook the onion, celery, garlic, and thyme together in the hot oil until the celery is soft, 7 to 10 minutes. Add the black beans, 4 cups vegetable broth, and the cumin to the pot; mix to combine. Stir the white beans, the remaining vegetable broth, and the sage into the mixture; bring to a gentle boil and simmer 30 minutes.