Any Nihilists here?

[Belacqua]

What is the natural end / teleology of being human?  Reproduction?

Naturally there was a lot of debate about this. For Aristotle, it was contemplation of the true. For Christians, contemplation of the True enlarges to heaven. 

For less ambitious thinkers, it's just a healthy life in which you use your talents as best you can.

And why not make it more specific, like the telos of me specifically?

For those who see all humans as having the same telos, your goal would be the same as everyone else's -- though it might express itself differently due to differences in circumstances. But yes, the actual life you lead, while fulfilling your telos, might look very different from that of someone in a completely different time and place.

And why base it around humans, rather than base our morality around the telos of spiders or rocks?

Well, this is just talking about people. Spiders have their own telos. 

That said, I personally prefer a society aimed at making things better (in terms of a rough negative preference utilitarianism) for all.  Sounds good.  I subjectively value that.

Yes, this is why I think it's compatible with nihilism. I'm perfectly happy conceding that "this is just the way I prefer it." There is no proof that a peaceful healthy society where people can live long and prosper is best. But I'll bet you we could get a good bit of consensus on the general goals.