Google Voice assistant without android auto
 

I know Android Auto works pretty nicely and you can hold the voice command button to trigger Google Assistant when connected to AA however I don't always want to plug in my phone for short trips.

Has anyone figured out a reliable way of triggering Google Assistant voice commands through the car interface when just connected over bluetooth? I press and hold the voice command button the steering wheel but it doesn't always work properly. Really it just triggers the Google Assistant to listen the first time each connection. Would be handy to initiate calls/change songs etc via Assistant voice commands when not using wired Android Auto

That's how it's supposed to work. Long press activates the Google Assistant. There's not much else to it. If it doens't work reliably then your phone somehow isn't quite getting along with the MMI. I have no experience with Android, but Siri on the iPhone works the same way. You can also just say Ok, Google and it should come on as well. I mostly just do the "Hey Siri" command. Works almost better than the long press.