Forrest (@Pfhorrest) if you get a second, could say a little about Leibniz's answer to the PoE, and what he had to say about heaven? — frank
I actually don't remember anything in Leibniz specifically about heaven, and some quick Googling to try to jog my memory mostly finds people asking similar questions (how does heaven fit into his solution to the PoE because it seems like it shouldn't) and commenting on how Leibniz' metaphysics isn't really trying to mesh perfectly with Christian doctrine, plus one paywalled article I can't read on Leibniz' view of purgatory, so it's possible that Leibniz never really said much about heaven per se and that's why I don't remember anything about it. Given his identification of souls with monads and the location of monads in the actual world, it seems like any kind of afterlife besides a rather naturalistic sense of reincarnation wouldn't be very compatible with his metaphysics.
As for his solution to the Problem of Evil, it seems to me like you mostly covered it. Leibniz thought that God could only create a universe that was logically possible (rather, that only certain combinations of things are "compossible" in the same universe, and all God can do is pick which such combination of things to make actual), and that God being all good would necessarily have created only the best of all of those possible worlds, so the actual universe that exists must necessarily be the best of all possible worlds, and whatever evils may still exist in it could only be done away with by instead actualizing a different possible world with different and still greater evils. Personally I don't find it very convincing, just kind of an abstraction of the usual free will type of theodicy ("God did the best he could, any better is logically impossible").
Plus, yeah, it seems to suggest that there can't be any kind of heaven that's better than this world.
As to the actual topic of this thread, I more or less agree with the thesis of the OP, and that's basically the reason why I don't have kids. I can't conscience bringing new life into the world when I can't be reasonably sure it would be a good life. I advise most other people to make the same choice, and I would have advised my parents to do the same. Basically only rich people should be having kids, and not even all of them. Which is not to say that everyone else should be prohibited from it, because while creating new life is risky (for that life) it's not guaranteed harm; nor is that at all to disparage the poor at all (of which I'm a part myself, hence my decision). Rather, it's an abject tragedy that this has to be advisable for so many people. Following that advice en masse would do something to ameliorate that tragedy for future generations though, much like the Black Death in Europe, as tragic as it was, did much to elevate the socioeconomic status of the survivors. To wit: if we poor don't make more poor people (by breeding), the rich who depend on us will eventually have nobody to depend on and will have to fend for themselves, and those future generations will be forced to be more egalitarian.
I do think that humans in general should continue to procreate though, even if we were all on hard times; it's just because of the fact that there's really no danger of us not having enough kids at this point in history that I can advocate for most people to not have kids. The reason I would advocate for humans in general to keep procreating even if life sucked for everyone right now is the same reason I advocate that individuals having hard times don't just kill themselves and end the suffering now: because it can get better. I have hopes that humanity can create a future world that is not so full of suffering as this one always has been, and in order for that to be worth doing, someone needs to be alive in the future in order to enjoy it.
None of this contradicts the Problem of Evil at all because God is supposedly omnipotent and so, if (he existed and) he wanted to create some beings to enjoy life, he could just create a world that was entirely enjoyable and had no suffering in it, and wouldn't have to just create life amidst suffering and hope that things got better eventually, like we do.