This Hario Mini Mill Slim coffee grinder has the potential to produce consistent grinds at most settings but it requires modification to produce acceptable results. The problem lies in two places: play between the metal shaft and the plastic housing, and play between the inner burr and its mount.
Stabilizing the shaft
The goal here is to fill the space between the housing (A) and the shaft (B). The gap between A and B allows for 1-2mm in burr play, resulting in inconsistency, especially at coarse settings.
I use a section of plastic from a milk carton. Tape works as well, and it is much easier to install, but it will wear down more quickly.
Cut a piece of plastic to roughly 3x8 cm, roll it tightly around the shaft (B), and jam it into the gap between A & B. If the plastic shim is too large trim it down until it fits. It took a lot of wiggling and pushing to get the shim to settle into the gap. You can actually see the top of the shim in the photo above between the housing and the shaft. Using tape wrapped around the shaft instead of using rolled plastic is much easier to insert.
Tightly roll the shim around the shaft and force both the shaft (B) and the shim down into the housing (A), moving the shaft and shim as one unit.
The shaft should now be rock solid.
Stabilizing the inner burr mount
This fix is easy. The plastic mount above normally wiggles when snapped into its place in the inner burr. One or two wraps of scotch tape around the plastic mount will eliminate most of the play here.
At this point the slop in the burr/shaft should be completely absent. Grinds are more consistent and results are more repeatable.