As a hobbyist music producer with an interface always connected, that microphone indicator is so annoying and unnecessary. I can't believe it can't just be disabled outright. I like macOS but it's too opinionated and some of those opinions SUCK.

Yeah I can see that being a source of annoyance for situations like yours. However, I welcome it from a privacy perspective. The indicator alerts the user if some nefarious application covertly enables the microphone.