I'm currently using an app called Choosy which allows me to hold down the Command key while clicking a link (in any app) and have a non default browser load the link. This is helpful when I'm running two browsers in tandem. Is there a way to make KM do this so I can get rid of Choosy?
Nyet.
That's a pretty low-level hack, and Keyboard Maestro can't do anything approaching the convenience of Choosy.
The best you can do is change the default web browser, and the macOS makes you confirm the change every time.
You can probably find several threads on the forum by searching for “Choosy”.
-Chris