If we don't allow potential people to carry weight, and if we are preference rather than hedonic utilitarians, then the only thing we are checking when deciding to create all these new people is whether or not existing people would prefer to do so.

That's Peter Singer's view, prior-existence instead of total. A problem here seems to be that creating a being in intense suffering would be ethically neutral, and if even the slightest preference for doing so exists, and if there were no resource trade-offs in regard to other preferences, then creating that miserable being would be the right thing to do. One can argue that in the first millisecond after creating the miserable being, one would be obliged to kill it, and that, foreseeing this, one ought not have created it in the first place. But that seems not very elegant. And one could further imagine creating the being somewhere unreachable, where it's impossible to kill it afterwards.

One can avoid this conclusion by axiomatically stating that it is bad to bring into existence a being with a "life not worth living". But that still leaves problems, for one thing, it seems ad hoc, and for another, it would then not matter whether one brings a happy child into existence or one with a neutral life, and that again seems highly counterintuitive.

The only way to solve this, as I see it, is to count all unsatisfied preferences negatively. You'd end up with negative total preference-utiltiarianism, which usually has quite strong reasons against bringing beings into existence. Depending on how much pre-existing beings want to have children, it wouldn't necessarily entail complete anti-natalism, but the overall goal would at some point be a universe without unsatisfied preferences. Or is there another way out?