I have a similar issue, because in my opinion, soap is poorly designed. No matter how you use it, you will always wind up with a nearly useless, limp soap core that is hard to pick up, hard to hold, and in constant danger of being washed down the drain, where it will succeed as a drainage obstruction for weeks.
To solve this problem I came up with the idea of a plastic soap core, inserted when the bar of soap is manufactured. When you reach the core, you throw it away. But this is wasteful, even if the core were made of recycled plastics. Plus, it adds cost to the soap. So I came up with the following design:
The soap is made with a hollow core, open at one end. When your bar of soap gets small enough, you open a new bar, slip the old piece into the slot, and go about your business. Nothing is wasted, and the cost of manufacture isn’t any higher.