Spinoza.

I feel like you've been talking to our detractors too much. You can just read Concluding Unscientific Postscript or Either/Or if you want to get into Kierkegaard.

If you're interested in Kant and Lebiniz, you should read Difference and Repetition by Gilles Deleuze. Read it, read who he references, and then read it again. It's very good.

If you'd like to generally learn about Western Philosophy, aside from Plato, you should also read Aristotle. You can just go from Aristotle to Descartes, and people often do, but there are some academics primarily concerned with research who would lament this to absolutely no end. I wouldn't worry too much about it, though.

Honestly, if you're interested in Existentialism, I would do this as succinctly as humanly possible and go straight from Thus Sparch Zarathustra to Being and Nothingness. It's not that the other Existentialists don't have anything to say; it's just that that is just what you came there for. Do what you desire and don't believe that you need to endlessly read before you can appreciate reading what you really like.