How do I set up my command key to turn on the microphone for diction ?

Dictation in macOS 11.6 is designed to recognize the different shortcuts that it offers you in System Preferences > Keyboard > Dictation > shortcuts. Again, with Big Sur, Dictation is now enabled via System Preferences > Accessibility > Voice control, which works more reliably than omitting that step, and attempting to separately launch and communicate with dictation in an open application's document.

You'd want to use some other key in combo so you didn't loose all the other commands that use the CMD key.

Keyboard Maestro...

http://www.keyboardmaestro.com/main/

Might try Karabiner...

https://pqrs.org/osx/karabiner/ 

You can also easily make custom keyboards with...

http://scripts.sil.org/ukelele

Thanks to Old Toad…

You can do what you want with Keyboard Maestro and a whole lot more .