Combine squash, onion, bell pepper, garlic, broth, black beans, pinto beans, tomatoes, chili powder, cumin, paprika and salt in a 6-quart slow cooker. Cook for 4 hours on High or 8 hours on Low. Mash some of the squash to thicken the broth. Serve topped with avocado and garnish with cilantro, if desired.