My SPDs are two sided...SPD on one side and a regular shoe side also. I can ride in Crocs if I like, bulky hiking boots and bike shoes too. I totally agree with getting any load you carry as low as you can without heel strike or other interface. 15 kg as a load seems to be quite a bit unless you are camping out and cooking your own food. Perhaps you could pare that down a bit? Something that clicks on and off when you arrive at a destination or feel safer with your stuff with you when leaving your bike for a meal or whatever is also a big plus. And as we all have said, a good fit/ ergonomics is critical.