Hi all. I have a raised deck with a land slop of approx 12% under it. I'd like to make that usable space (more for storage than anything else) 25x15ft approx.

it starts at about 7 ft high (existing patio) to 4ft at top of slope over the course of 25f. I've started to dig it out, and my current plan is to make it 7 foot clearance throughout with proper pitch from house and put on fabric with gravel over it. The problem then becomes with preventing erosion.
Original plan was to do a retaining wall block wall, but that's a lot of work and expense for just storage space.
Before doing that, I was wondering what suggestions, if any, you all have. There's probable other options that would work just as well for the function, that I just haven't thought of, so suggestions are welcome.