I couldn't find almost any tutorials on how to disassemble or service this particular eBike motor (Bafang M400), so I just started removing screws and pulling out parts while taking a ton of pictures.
I found two dirty, rusty bearings that I cleaned and re-greased, along with all the gears, and then put it back together. Not only did I manage to avoid breaking it, but the noise seems to be gone as well. This was by far the most intimidating bike repair I've done so far. I've serviced every other part before, but I hadn't dared to touch the motor until now.
It also turns out that the motor is much better sealed than I expected, so I'll happily continue doing deep river crossings with the motor half submerged in the future, just as I have done until now.
Hex keys, philips and flat drivers, hammer, steel picks, lock ring pliers, 5mm (I think) bolts to push out bearings and the metal casings and different size sockets to push bearings back in
Yay for non-proprietary fasteners! Which part of the motor needed the hammer?
All the bearings are push-fit into the metal casing so they need quite a bit force to come loose as well as to push them back in place.