Well, I managed to shape a couple stones to a tight point, and I made myself a needle shaped burnisher. Combined with the careful hand use of gravers as scrapers, and more patience, I got to a better finish in the inside corner.
There are still issues here obviously (background flatness, and the rounding of the foreground due to my indiscriminate use of sandpaper without hard backing), but I was focusing on the tight corner and that's a lot closer to what I want (this is just a practice piece).
Additional tips are still welcome