The next step is a dog flap! My Zoe used the cat flap until she became too big so I bought a dog flap which I thought Pippin (Siamese cat) would find difficult to use because it was so heavy but he used it straight away and it took a whole morning if enticing Zoe through the larger hole with a Bonio because she felt insecure! When Jamie came along as a 15 weeks old pup, he just followed Zoe who effectively toilet trained him.
Forgot to say well done to Bailey - what a clever little girl.