... and I "fixed" it: I ended up opening it up, pulling out the whole wheel/encoder wheel/PCB assembly and, um, just blowing on it? Might have had some cat hair or something in it, maybe? Regardless, I kept doing that until I was getting even response nose-up and -down, then reassembled. On reassembly I don't think I kept the axle alignment perfect so the wheel resists turning a bit more now, but frankly I actually like it better - before it spun freely for most of a revolution, which didn't really feel right (even though I've never set a finger on a real Cessna wheel) ...
So, there you go. Trim wheels are prone to cat hair interference. Who knew?