Solved my problem - posting here for reference in case anyone else runs into it. I ended up buying this $10 motor from Amazon for the E-step. The wiring on it is indeed not what the i3 Plus (in my case monoprice maker select plus) wants.

The i3 Plus originally used a motor from Moons (https://www.moonsindustries.com/.../nema-17-standard...) that had its wiring pinout go Black (1), Green (3), Red (4), Blue (6).

This motor instead goes Green (1), Blue (3), Black (4), Red (6).

So the fix I did was to change the pinouts on the connector. With the standard cable that comes with the motor, it will be Black (1), Green (3), Red (4), Blue (6) where it connects to the motor. On the other side of the cable, I removed the connectors and reinserted them as Green (1), Blue (2), Black (3), Red (4). (Looking from the back of the printer, the far left connection is #1.)