Miss Marple's Final Cases

I'm curious whether these stories were written with the idea of being the last for Miss Marple or if they were selected later. Did the author have a specific ending in mind for her?

The stories seem to give a deeper insight into her personality and worldview. I wonder if this was intentional or a result of her being portrayed in her final cases.

The collection feels like a fitting farewell, but I'm curious what drove the decision to compile these stories as her 'final' cases—was it Christie’s idea or the publisher’s?