Today, I mostly investigated and tweaked the tasks in my noxfile to make them easier to reason about in the context of parallel execution. There are just a few tweaks left before I'd be comfortable running all of this code in parallel. The main thing missing... is a runner.
I wrote up a short summary of what I want and what I've looked at elsewhere, earlier today, but the shorter summary is that it seems like what I want to do falls just outside the intended usage of every tool I've looked at. Writing up the specific things I want kind of got me thinking, like, "Hey, these aren't complicated features! I could make this!" (I am conveniently eliding the basic plumbing of a task runner, like "finding and loading the configuration file".)
In any case, if nobody suggests something that works for me, I will probably roll my own, and suffer in some way that I am completely failing to anticipate. Anyway, I won't do that right now, because it's getting late.