Wouldn't it be sensible to fill my whirlpool with my domestic softener? I would have a water hardness of 6 degrees German hardness?
This will not work or only to a limited extent.
Almost all softening plants work on the ion exchange system. In the softening plants there is an ion exchanger granulate in a container. This is a special resin containing sodium ions. The water flows through the tank. The sodium ions from the resin are absorbed by the water and the calcium ions from the water are transferred to the resin. The resin only has a certain flow capacity before it is saturated with calcium ions.
In a normal plant this is often only 300 to 400 litres. As soon as the resin is saturated with calcium ions, it must be circulated with salt water so that the absorbed calcium ions are dissolved from the resin and enriched with sodium ions. During this time, unsoftened water would flow through the softened system. In other words, the first 300 to 400 litres are softened. The rest is not. And then you have much too hard water in your whirlpool. In case of doubt you have to consult the manufacturer of the softening plant how high the continuous capacity of the softening plant is.