I was wrong! I just put a long stick against the starter motor and whacked it with a hammer a couple of times and sure enough, it started!
And yet I'm stubborn enough to still think it's not the starter. When I pulled the starter out half a year ago and bench tested it, it spun up like a champ, almost jumping off the bench even though it was tied down. There was no hesitation in starting. Whereas when it's in the car, it sounds like it's struggling to get current. Plus, some other electrical components are behaving weirdly--stereo display is dimming, power window switch produces a click from somewhere under the dash or behind the firewall (relay?), and the power locks are weakly struggling to depress all the way. I have a feeling it's hidden corrosion. There have to be a million connections coming off the positive battery terminal, and I highly doubt it came from the factory looking like it currently does.
Any advice on how to best test or replace the terminal connections?