When I took apart the transmission and motor, I found the source of the annoying squeak at low speeds: my coupler was falling apart! I thought I had used Loctite to keep the bolts together, but I couldn’t find any evidence of it. The coupler was loose on the motor shaft, and the clutch plate pieces had flung apart from the high RPMs.
At least nothing was broken. I’m not going to use the same coupler again anyway, so it’s really not a problem anymore. This time the shafts will be connected by a simple shaft coupler. I’m going to make sure to use Loctite this time though.
I’m glad to have found the source of the squeak though. It was getting annoying to hear squeaking at low speeds. Having a perfectly silent car is the only way to easily hit pedestrians in parking lots.