Cut the trimmed eye of round with the grain into strips between ⅛ and ¼ in thick; if the roast is too wide to handle, split it horizontally first. Keep the pieces as even as possible so thin ends do not become brittle before thicker pieces finish drying.
Whisk the dark brown sugar, soy sauce, Worcestershire sauce, smoked paprika, meat tenderizer, black pepper, red pepper flakes, onion powder, and garlic powder in a medium bowl. Continue whisking until the sugar dissolves and the marinade looks smooth and evenly seasoned.
Coat the beef strips thoroughly in the marinade, then cover the bowl or transfer everything to a large ziplock bag and refrigerate for at least 12 hours or overnight. Toss the strips or turn the bag once or twice during marinating so the seasoning reaches all sides.
Cover two baking sheets with aluminum foil, place a wire rack over each one, and heat the oven to 175°F (79°C) with two racks positioned near the center. The raised racks catch drips below the meat while leaving space for air circulation around each strip.
Lay the marinated beef on the racks in a single layer and dry for 3 to 4 hours, switching the pans from top to bottom and front to back halfway through. At the midpoint, the strips should smell deeply savory and look darker, with no overlapping pieces blocking airflow.
Remove a test strip and let it cool to room temperature before judging the texture, then seal fully cooled jerky in an airtight plastic container, Ziploc bag, or airtight glass jars. Finished jerky feels dry and leather-like, bends with resistance, and remains chewy with a little tenderness.