Hi,
I want to change default audio output on my MacBook from built-in speakers to HomePod Mini on my desk via keyboard shortcut.
How can I do that via command line / AppleScript / Keyboard Maestro?
Have you ever looked at SoundSource from Rogue Amoeba? As soon as you want to reliably manipulate things like this, especially if you want to avoid using timing and mouse clicks, etc. third-party specialist utilities are often worth their weight in gold.
I don’t have this app myself as my audio output needs are very simple, but I use a number of their other audio apps (and have done for a decade or so), and their products are very good.
You may also be able to find other apps that offer options that might make your request that bit easier to deliver on, but SoundSource is the one that I always hear mentioned.
Another SoundSource (and Rogue Amoeba) fan here, too 
It may be overkill for what you want, @matt08, but if you do choose to invest in it, I use a combination of Shortcuts and Keyboard Maestro macros to do just what you want (and more!)
Best wishes,
Steve