Slope angle decorative gravel

On about 270cm there is approximately a 180cm height difference. Two "terraces" will be created with retaining walls. However, I want to keep the walls as low as possible and instead hold the "terraces" sloped with gravel. The question is therefore, how sloped does it still look nice, if you lay white gravel and possibly a few plants, and up to what degree do the stones (about 25mm) stay securely in place. I was thinking of max. 2 x 75cm walls and the rest with a slope, or even better just 2 x 60cm and the slope a bit steeper. With 2 x 60cm that would be about 60cm height difference over 230cm length (270cm - 2x20cm (wall thickness)). I hope it is somewhat clear, maybe I can upload a sketch...