Do I really need a dedicated touring bike, or can I tour on an endurance or gravel bike?

It depends on your touring style. For bikepacking with frame bags and seat packs on mixed surfaces, a gravel or all-road bike works well. For traditional rack-and-pannier touring with heavy loads on paved roads, a dedicated touring bike is significantly better due to its rack mounts, stable loaded handling, lower gearing, and longer chainstays for heel clearance. If you plan to do more than one or two tours, a dedicated touring bike is worth the investment.