Looking at your latest renders/RL pics of the assymbled stepper motor mount and I had some questions and maybe ideas. Posting here since I was not an initial kickstarter backer (just a preorder =p) I cannot post there.
First the question… In the RL pic showing the stepper motor in the mount with the pour over cone… can you adjust the position of the lower left guide pulley dynamically? Basically I am wondering if your system can be adjusted to ensure good belt tension. If that could be adjusted along that edge you can take up any slack (what appears to be slack in the image). Perhaps on a spring loaded slide to keep it automatically under tension.
Which would basically be my suggestion… that or perhaps add 1 more guide pulley in the lower left blank area that can be adjusted to compensate for slack in the belt. It would also allow you to loosen it easy to change/modify. Below is the pic in reference.