Thank you for the responses and the help everyone! I've got all motors working now, and should be able to finish the rest of the machine from here without any issues.
The primary two issues turned out to be the bad drive mapping (I was using P4 instead of P5) and the flipped ENA input. Once I changed those two things, the larger motor worked perfectly. Thank you for your help @dc42! (and for creating a good product in general, this thing has been a pleasure to work with)
@JoergS5, thank you for pointing out the high stepper current. I had originally thought that my larger motor needed 4A, but your comment made me go back and double-check - turns out 2A was correct after all (and thanks to you I didn't even have to burn a motor to find that out).
Incidentally, this setup actually does have two stepper motors for extrusion, that bit wasn't a mistake. It's a clay-extruding printer (Cerambot), with a larger motor pushing the clay out of a large syringe and through a feed tube to the printhead, and a smaller stepper at the printhead itself driving an archimedes screw to extrude the clay onto the build plate