  Mitigation of beam backgrounds via collimators is critical for the success of
the Belle~II experiment at the SuperKEKB electron-positron collider. We report
on an improved simulation methodology, which includes a refined physical
description of the collimators and beam pipe, our first implementation of
collimator tip scattering, and in which the existing beam particle tracking
software has been embedded into a new sequential tracking framework. These
improvements resolve longstanding discrepancies between measured and predicted
Belle~II background levels, and significantly reduce the computing time
required to optimize the collimation system in simulation. Finally, we report
on collimator aperture scans, which confirm the accuracy of the simulation and
suggest a new method for aligning the collimators.
