How to always use macbook microphone with Bose QuietComfort 35?
I still really like @aaplmath's suggestion (and that may be the best one going forward), but I also just stumbled on this solution that requires no coding (AppleScript/Swift/ObjC/Other) in High Sierra, and it's super easy to configure.
Please note that the answer was in the comment section of the article and it seems to work on High Sierra (I'm on 10.13.6), but I don't know if this works on Mojave and beyond.

Open the standard &quot;Audio MIDI Setup.app&quot; utility, and click the + at the bottom to create an Aggregate Device. Then select the Internal Microphone as the only component in this aggregate device and select it as the default input. For some reason this prevents Mac OS X from selecting the Bluetooth Microphone from now on and it will stay on the internal one when you reconnect.
After creating that aggregate input, go to your usual System Preferences -&gt; Sound -&gt; Input (tab)

Source: http://ssrubin.com/posts/fixing-macos-bluetooth-headphone-audio-quality-issues-with-hammerspoon.html#comment-4018174387