I use superwhisper (but maybe should use this!). I use karabiner elements to map the microphone button (f5) on my mac to be f20. I than tell superwhisper it's shortcut key is f20, works super well!
Hey, I use Karabiner too—love how customizable it is! But I mainly use it for emulating the the PC (Windows) keys setup, I was a long time Windows user before switching to macOs.