Don't get sidetracked by brand loyalty or think you need to match your road bike's price point. At this budget, you're getting solid entry-level gravel rigs from all three brands you mentioned, so picking between Trek, Specialized, and Canyon really comes down to what's available in your size and what feels right when you test ride it.
Tire clearance matters most for your situation since you're jumping from road to dirt. Look for something that'll take at least 45mm tires comfortably - ideally 50mm if the frame allows. That cushions rough terrain way better than cramped road geometry. Frame material is honestly secondary; aluminum at this price point works fine and won't hold you back. What you should actually prioritize is component quality, specifically the drivetrain and brakes. A decent groupset with reliable shifting and hydraulic disc brakes (or good mechanical ones) will save you headaches and money on maintenance. Skip models with the cheapest Shimano Claris or equivalent - step up to Sora or 105 level if you can find it in budget.
Run through local shops and test ride what they have. Canyon's online-only model means no bike shop support if something goes wrong, which matters when you're new to gravel riding and might need help sorting out setup issues. Trek and Specialized dealers can actually help you dial in tire pressure, suspension forks if applicable, and geometry tweaks. At $1500, you should walk away with something that can handle packed dirt, some technical sections, and maybe light singletrack without feeling sketchy.