Good news for those who were not able to solve this issue of the steering wheel button triggering Google Voice Search instead of the much more useful Google Assistant. I found it and I can now open my garage door by just pressing the steering wheel button and gently asking Google Assistant to do it for me 😁
DUDU6 on Mekede M6 Plus 3D (I guess it should be the same for everyone though)
Settings
System
Android Settings
Box Set
Voice Package Set
Select "Assistant" and enjoy (or any other app that you may want to open with the steering wheel button)
EDIT: Now I would love to kill it by pressing the steering wheel button again instead of having to touch the screen out of the assistant window to close it. And would also be great to activate a dark theme but I can't find it anywhere for this app. (this last one was just a matter of switching on the "Sync with Android Global Day and Night Mode" in Home > Day/Night. It is now light mode or dark mode depending on the car lights)