My hugelkultur space settles A Lot every year. I am always topping it off.
My native soil is opposite yours: one foot of clay and then rocks. Rocks till the center of the earth. When it is dry it is like concrete and when it is wet it is like baby poop. So, the heavy heavy always decomposing organic mulch is the best for my home property. Unfortunately the public farm is almost nine acres and I cannot mulch all that. My favorite store bought mulch is wheat straw - pretty cheap.
I've brought in at least 5 pick-up truck loads, and it just disappears into the sand after a couple of years. I'll keep getting it, but I think I need a few dump truck loads to make a dent in my soil.
I also have my compost pile, and all the leaves I collect and shred. I have folks that will drop off bags of leaves here for me.
It's just a forever kind of thing. I guess I should have started amending the soil 40 years ago when I had the chance. Youth is wasted on the young.
/johnny