In Audacity 2.1.X there is real-time adjustment of effects , e.g.
The ToneBooster compressor with the soft knee may be more-like the curve you are looking for.
Again there are free plugins , e.g. FloorFish , which is an expander (only).
Chris’s Compressor [free] will also work as an expander (only) if you use a negative value for the compression ratio.
But I think those plugins only expand above a threshold, so will make your unequal-volume-voices problem worse.
Steve made a [free] plugin to make the voices in a conversation a similar volume , called “levelSpeech.ny” …