Do I still need a dedicated hub if I already own a HomePod mini or eero?

Often no. Across the reviewers we read, both Apple's HomePod mini and Amazon's eero hardware double as Thread border routers and Matter controllers, so households already inside those ecosystems frequently don't need a separate box. A dedicated hub like the Aqara line makes more sense when you want Zigbee bridging, local automation, or PoE-powered redundant Thread routers.