Last fall I planted a couple dozen fruit trees. I rented a small skid steer with an auger, 24" & 12" bits. I started with turning a ~2' deep 24" hole and then went back into the center of the hole with the 12" bit as deep as I could go without disturbing the mound the 24" created. I added some worm composite when I planted the trees.

The trees have absolutely taken off this year, far beyond what I was expecting.

That's a good point on the ground settling I didn't think about. I haven't seen that happen yet, but it doesn't mean it won't.