The problem with motion sensors is that breezes will set them off as well as clouds passing over the sun. So it’s just really hard to get them to work well outside. Not impossible, but very difficult. And people will generally use several in a zone set up to reduce the number of false alerts.
As an alternative, pressure Mat might indeed work very well for you. They’re available for weights as low as 5 pounds so you could easily select one that your dog would trigger when they stood on it. These are often used as doorbells on houseboats in a similar use case.