I finally received the Assistant Driving Mode a few weeks ago, either with a Maps update or Google app update (or a combination of both). It works OK for me, you just have to go into phone Settings > find the built in Driving option in Settings and make sure it's set to Do Nothing and not the default: Enable DND.
I'm not happy that Google implemented this to only work while navigating with Maps. I know my route to work and home like the back of my hand Google. I was really hoping (based on their blog post announcing this in 2019.....it took 2 years Google?) that this would be something we could enable any time we were driving (like the old Moto version). I guess it's a start and hopefully Google will give us the ability to turn this on without navigating with Maps.