Heat a large skillet over medium-high heat. Add ground beef and onion; cook and stir until beef is browned and crumbly, 5 to 7 minutes. Drain and discard grease. Transfer beef mixture to a slow cooker.
Cook bacon in the same skillet over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Transfer to paper towels to drain. Crumble into small pieces; add to the slow cooker.
Add baked beans, black beans, light and dark kidney beans, great northern beans, sugar, mustard, and soy sauce to the slow cooker; stir to combine.
Cover slow cooker; cook on Medium until flavors meld, about 4 hours.