Well anytime you pour concrete you have to put sand down first and tamp it. if not you slab will only last a couple years, If you need any help when i comes time to do your floor, let me know, I have poured many a concrete slabs
This is where the issue comes into play.
If I put the dirt back in, it will take a couple of years to settle with the root debris.
I work at a power plant that has bottom ash for free, so I' might get a guy with a dump truck to bring me a load.
Water it down totally and it should settle when the water goes away with lots of packing by my tractor.
I hope. How does this sound to you?