The cantilever staircase does not hold, the stair builder is not a staircase builder

The anchors have clearly failed here. I have had very good experiences with Hilti adhesive anchors. Follow the exact processing instructions. You won't pull that out. What kind of concrete is that? How old was it at the time of installation? The anchors were pulled out without any load? Or did someone jump at the end of the cantilever?

Who would dare to carry furniture up there, every step can come loose and crash down with furniture and two carriers. Is the concrete wall stable enough, the craftsman could already notice that when drilling.

Now I understand, the right photo. That is not your staircase. Rather an example. You don't have such a plate on it? Take a photo where the entire staircase can be seen. Without such a plate, the tensile forces are so great due to the lever effects that no anchor can hold. Hasn’t a structural engineer calculated or checked that?

Roughly calculated, about 40 kN per anchor act here under static load. With the plate, it is only about 4 kN.

Only involve a lawyer if you have absolutely no idea about contracts.

I suspect this is the case here insofar as choosing such a genuine fool would typically be comorbid.

Currently, we are considering enlarging the bearing surface with a sheet metal welded to the current steps and additional glued threaded rods, see example photo.

It would have been sensible to show the condition from multiple perspectives of both the first and the second attempt(!) instead of just one picture of the second attempt and an example photo of a hoped-for suitable alternative solution from elsewhere. However, there is enough to suspect that this is a premiere for this "stair builder."

Is this how you wanted the steps? But no architect planned that. How is the railing supposed to be installed there?

I already suspect that the "load-bearing" concrete wall was not planned (and pre-equipped!), but that the OP comes from a cunning place (near Schilda), had the house "planned" by a general contractor’s lackey, and immediately excluded the staircase as "client-supplied" (possibly because the general contractor knows its limits and is aware it has no experience with this special request). This would then be a textbook example 1a (no, actually even AAA+) of utterly ignoring the 11ant Steinemantra.

"Load-bearing" wall, special mounting elements (there is surely something nicely expensive for that in the Schöck pharmacy alone), and the "actual" staircase including the railing (see above) form a system that only daredevils/hazard seekers try to separate. "Little sins are punished by dear God immediately"—the combination of an excellence-detail special request and cheapness is probably a small sin of the highest order. In this respect, I cannot suppress the honesty that—even though I still consider accepting the job by the bungler punishable—my sympathy in such a case is not only minimal but actually negative.

The whole thing cost him "only" half, which is still a lot of money.

You can assume that financially he has already spent his powder on your order. First check if there is anything to gain from it. Why throw good money after bad?