I have considered using some terracotta pottery to add some additional support for a steeper slope in my 85gal. Both square and rectangular pots are readily available. I expect it will end up with more of a terraced look than you are wanting though. I am 100% sure that it's fish safe, since I have tons of it in with my goldfish.
While looking at some ancient city wall constructions ( I study archaeology ) I thought it might be an idea to use pebbles of 2cm radius at the bottom then layering those with smaller pebbles as you go higher and finally covering them with moss or aquasoil.
To prevent the soil from sliding off you could try making a grid of some sort or by terracing.
I am going try this idea using screen with .25 in mesh. Thinking of making inner of high hill with wire mesh in-compassing rocks. Will use spray foam to cover cracks.
great stuff expanding foam is a safe project, I read it somewhere, but I would double check before I tried it in one of my tanks. Use the great stuff to build the base shape. When it freshly applied force substrate or larger rock pieces into it. It will harden and the first layer of substrate will be attached. You could use a drywall saw to cut in pots or to shape it.
Phil Edwards used great stuff to build a moss wall here.