Tough problem..... I can see why your frustrated. My best advice is to build a retaining wall, level the loose soil up against the wall and plant on top of that. I can't tell how high the slope is but I would build the wall almost to where the ground starts to level off. IMO nothing is going to grow there without help..... I don't see any "weeds" even trying too. I see a lot of tree roots in there too and they will use any available moisture first. I would suggest a creeping blue rug juniper planted along the top after you put in the wall. It may sound like a ton of work .... but if you do it right the first time you won't have to do it over.

Do you have a wet season? If you can get sword fern established during a wet season, they handle droughts pretty well. And we have almost sheer hillsides here held in place by them. But they do need wet part of the year.

Retaining wall OR get someone to come even it out so that it is more of a longer, less steep slope. Vinca major survives in both sun and shade and survives when it is dry...but, needs to be planted in a moist, cool time of year, preferably fall...and watered a bit, in some cases, the first year. it is evergreen and will look better, once established, if it is mowed or weed-wacked back once a year. I pulled up a few sprigs from a burned down house that was long abandoned. It is now covering my back yard. Once a year, I help it by moving a few pieces in the bare areas and/or burying the trailing stems to make the groundcover thicker. I have seen ivy used, too, but that climbs, so means more upkeep.