Yeah it’s rough here, that picture has about 6” of unscreened loam under it before it’s pure clay basically. And I didnt put very good soil in it at that point. Making up for it now.
I think if you just turn 8” or so of whatever is there, so a spade depth and mix it with some good compost you’ll be at a great start, and if you go up 12” you’ll be at a decent 18” if you stay down from the top. Turning that 8” is already going to increase the volume by up to 30% so keep in mind when doing any calculations. Fill whatever is remaining with a 50/50 compost loam mix.