It's a gross hack but works for now. To prevent overly sensitive mic settings autotuned by the browser in web conferences, I currently edit as root /usr/share/pulseaudio/alsa-mixer/paths/analog-input-internal-mic.conf. Change in [Element Capture] the setting volume from merge to 80.

The config block as a whole looks like this:

[Element Capture]
switch = mute
volume = 80
override-map.1 = all
override-map.2 = all-left,all-right

Solution found at https://askubuntu.com/a/761103.