Right now Twitter isn't doing anything but the first part, which is why "I got mass report banned on Twitter" stories are so commonplace. They need not be, if Twitter would do their goddamn job.
At some point, in designing the algorithms, in instructing the human reviewers, and so on, you still need to agree on what abusive means. So there's no way around that.
As to the volume, I agree, one can deploy algorithms as a first filtering step. In this case, I don't usually trust humans to be smarter than algorithms, but I do trust humans to be better at abuse than algorithms are at recognising it—unless the algorithms have so many false positives that they're barely filtering at all.
I'm likely in the minority on this, but I'd rather have the control than trust others to moderate in my best interests for me.
But you have to read all 829 M to find the abusive ones—and you have to agree on what 'abusive' means.