I'm a bit late to the party, but I had a similar issue and found a solution that worked for me.
There is a product from rogueamoeba.com called 'Audio Hijack' that lets you take audio from any input (or directly from individual apps), perform processing and send it to an output or record it. It's worth considering getting Loopback at the same time as there is a deal on getting both. Both have free trial modes where it starts adding noise after 20 minutes or so but you can restart the apps for another 20 mins testing.
There is a 'Built-In Effects' block called 'Volume' that allows you to alter the volume and it has 'Overdrive' options to increase the volume by up to four times. Seems to work well for my Aftershokz OpenComm headset that's too quiet for me even at 100%.
If the volume from the mic is really low though, it might be expecting to be powered by what it's connected to, so it might be worth considering a preamp or an alternative mic that outputs at line level.
I hope this helps.