Have you tried a polyextrude sop?
Also I am not sure what the goal of this is and therefore I am not sure what is actually required. As far as I can tell from the screenshot, the “inbetweens” actually have thickness and are one connected chunk? I guess one way to achieve this would be schrinking each fractured piece, peak the outter faces out and then boolean those chunks with the original geo.
Or maybe instead shrink each chunk (while maintaining the outter face shape) and somehow creat the inbetweens from the inner faces…
Just some ideas.^^