It seems clear that the filter would act as a mechanical filter, but whether that is 40 microns or 5 microns, I find no reference.
If you keep the cartridge filter, maybe switch to a 200 micron, or just remove the cartridge altogether. This should still collect rocks if any. Even an empty housing might keep you from backwashing at 20 gpm-- the rate I think best.
When you add the media, wear that N95 mask for dust. If there is no dust, having the mask in place won't hurt. With lighter media, you let the initial water in slowly. Then you do repeated backwashes to be rid of fines. In some media, you soak overnight. Now with your heavy solid media, this may be unnecessary. Would not hurt.
Regarding a normal regen, the unit will backwash, washing out any collected particles.
I would consider having a way to measure the pressure into the valve during backwash. This could be with a boiler drain valve which can mount a GHT pressure gauge. You will be collecting and sharing data.
For the drain line, you might consider 1 inch rather than 3/4.
For backwash time, you will experiment, including noting what your well can keep up with. But I foresee multiple backwashes when the media is new.