Specifically this step:
“Line up the two arm halves and then press the two halves together. This is much easier said than done and this is the trickiest step of the build process.”
Is a real pain to perform, but there is a much easier way to do it.
Assemble the non motor half of the assembly with everything except the drive gear as shown here:
Note the location of the magnet!
Assemble the motor half with only the bearing under the drive gear and the drive gear itself. Do NOT install any other bearings as shown here:
(side note that the JST-HX pins on the back of encoder board should be clipped and covered with an insulating tape you can see it through the holes at the bottom)
Then simply invert the motor half and press it down (nothing can fall out because its all retained). Gently rotate the spool to align the gears and wiggle as needed to get the bearings to sit right.
Hope this helps someone else!