I tightened up the S3X cog carrier (using a tool, not my fingers), which will hopefully get rid of that “bzzzrp!” sound at low-speed high-torque right-pedal revolutions. Also made a tiny adjustment to the shifter tension, which was much much easier with the rear wheel off the ground.
I’m still viscerally uncomfortable with leaving a fixed gear bike in a stand, so I’m going to go put it on the ground now…