It sounds like if a lot of digging needed to be done in the creation of the slope, it might have a lot of subsoil. Our subsoil is grey clay, and where we had no choice while terracing but to have it incorporated in the topsoil, it has improved as Fairygirl says with lots of organic matter added.
Terracing certainly won't make things worse, and will allow you to add organic matter without it just ending up in a heap at the bottom! It makes moving around a slope, and planting it, easier too.
I'd work on improving the soil for a while before investing in plants. As well as adding manure or garden compost, consider sowing green manures for a season before planting. They can avoid it becoming colonised by weeds, improve the soil structure, give the soil microbes something to live on, and then at the end you can smother them or dig them in and they'll add more goodness as they decompose.
It sounds like you've had major excavations @ericalessard. Is it possible that it's subsoil on the surface of your slope now? That might explain why it won't even grow weeds. Mixing in organic matter should help to bring some life into it.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
If it's almost cliff-like you'll probably need to terrace it if you want to improve the soil and grow a wider range of plants. You can't really do anything to improve slopes steeper than 45 degrees, digging compost in would make it unstable and mulch will slide off. To plant up extremely steep bare clay soil you'd need quite aggressive spreading plants - which is okay but not necessarily very "pretty".
"What is hateful to you, do not do to your neighbour".
My sister terraced her garden with scaffold boards held in place with stakes, each terrace is 2-3ft wide and backfilled with topsoil. It's been pretty successful. She's not on clay but it's a similar principle.
"What is hateful to you, do not do to your neighbour".
Take a look at erosion control matting, once laid, provides pockets for retaining top soil into which plants can grow initially before becoming more established when their roots will provide the erosion prevention.
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Very expensive too
I live in west central Scotland - not where that photo is...
Terracing certainly won't make things worse, and will allow you to add organic matter without it just ending up in a heap at the bottom! It makes moving around a slope, and planting it, easier too.
I'd work on improving the soil for a while before investing in plants. As well as adding manure or garden compost, consider sowing green manures for a season before planting. They can avoid it becoming colonised by weeds, improve the soil structure, give the soil microbes something to live on, and then at the end you can smother them or dig them in and they'll add more goodness as they decompose.
There isn't an instant solution. I'm afraid.
I live in west central Scotland - not where that photo is...
E.g. https://www.abg-geosynthetics.com/products/erosamat-type-2.html