uneven L/R waveforms on my mixes lately

That’s not going to work. Instructions how to attach images to a post are described here.

If the tracks appear asymmetrical when opened in Audacity, then the problem is pre-Audacity.
( I also think you can rule out USB cables).

Normalize in Audacity could correct the problem …
Normalize Stereo Channels Independently in Audacity.png
RMS-normalize is even better, but it’s not shipped with Audacity: it’s a plug-in you have to install.
RMS Normalize.png