Theres this soil called topsoil - I've been researching lol its desinged for animals that would need as a substrate and it totally hedgehog safe !!
Pro rep dessert sand - desinged for reptiles - is a good safe sand to mix into the soil too, not one with added vits and minserals though just normal dessert sand
Or kids play sand that has to be baked first is also a good one to mix with soil.
I did research to find this for you. Try looking on online reptile shops near you to find it.
Most people do it so theres more soil than sand, but some do it so its 50/50
I love the idea of adding the gravel too.
When you have it all set up show me what it looks like I'd love to see.