Autodesk Sketchbook is slightly ‘arty’, but very powerful, easy to use and, best of all, completely FREE!
It is, and I'd already downloaded that application. I may have to play with the settings more, but it didn't seem to have any sort of scaling option, and I didn't see a canvas size option (though I'm sure one is quite likely to exist). Only the "pen" tool seemed to draw a crisp, clean line, and even then it wouldn't auto-join the two ends of the lines (I could zoom in and to this manually). It was pixel-based, too, it seemed. Not vector based.
It may end up being the best available, but it's probably not "great" for my niche purpose. But maybe none of them will be.
On the Mac, OmniGraffle Pro would work pretty well. But that's drawing with a mouse or a trackpad… not a pencil on an iPad.