What planting is suitable on a very steep slope?

At the bottom of the slope above the swamp area, I would choose semi-shady plants. The forest will cast shade.

You can sow the area with forget-me-nots, marigold, nettle-leaved bellflower, red campion
Ground cover creeping bugleweed, small periwinkle, bird vetch
Perennials wild garlic, true marshmallow, horse mint, meadowsweet, wood betony

upper half Big cowslip Red campion Meadow rue Siberian iris Columbine (currently producing seeds, be sure to ask. Blooms only in the 2nd year) Water scorpion thistle Meadow knapweed Here I would use all sorts of ground cover as previously listed

On top of the crown, you can plant a bird hedge or fruit or fence and let it grow over so that it fits your garden

in between a few large Solidärpflanzen from the list of Hampshire

This is a suggestion to let the tendency towards the ecological paradise grow. It costs money, especially if you take ready-made plants. If you get a hydrangea for free, then take it. Just like the daisy on offer. Hazelnuts grow fast and vigorously. Indestructible those things. The soil and location are suitable for many plants. It should just be quickly greened up.

A different stupid question: Where does the slope come from?
That is not natural soil.