Preheat oven to 350 degrees F (175 degrees C). Spray a loaf pan with cooking spray.
Mix venison, pork sausage, and onion in a bowl until thoroughly combined. Run water from faucet briefly over onion buns, squeeze water out of buns, and break into small pieces. Add pieces of onion buns to meat mixture.
Stir eggs, brandy, bacon grease, Worcestershire sauce, thyme, red pepper flakes, seasoned salt, garlic and herb seasoning, black pepper, and sage into meat mixture. Transfer to prepared loaf pan.
Bake in the preheated oven until browned and the juices run clear, 60 to 70 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).