Try Simone de Beauvoir's the ethics of ambiguity. It's deals with many of the concerns Neitzsche raises in the area of ethics and moralality and provides some convincing arguments in a short accessible book.

Maybe Adorno's Minima Moralia might be nice? It's well written, in the sense Nietzsche is well written. Can also recommend Walter Benjamin.

In terms of interpreters of Nietzsche, it's considerably more difficult, but Deleuze wrote a really good book on Nietzsche that's worth reading.

Deleuze's books on history of philosophy are interesting and approachable. Although they are more about his own philosophy over people he write about they're still very valuable. Deleuze gets bad reputation for his other writings as they are difficult to read but that's not the case with books on history. He also have books on Nietzsche so that might be a nice starting point. Other continental philosophers that you could try are Heidegger, Foucault or Bataille. Everything depends on what you're looking for, but if you're coming from Nietzsche and Stirner they might be interesting for you.

Edit: As someone else mentioned, Walter Benjamin is also very interesting!

Søren Kierkegaard could be a good candidate.

If you like depressed mood then have a look at Schopenhauer?

I think Dostoyevsky and Camus are more accessible.

Schopenhauer!

Personally, I like Albert Camus very much.

Like Nietzsche, he has a bad reputation for being moody and depressing, but the way I see it, he is really liberating and optimistic. A true humanist.