Accessibility Voice Control to Read a Navigate a Webpage
Android's built-in Accessibility really focuses on reading/speaking items on the screen and using gestures to move around.

For voice control, one possibility is Google's Voice Access app in the Play Store, and the intro tutorial does give a good example of the app's features, but there seems to be some problems with the form-filling (speech to text into a field) when I tried it just now. I also noticed that it uses small numbers for focus change which may not be desirable while driving.

Using an Accessibility app is pretty invasive from a permissions point of view so provided that you trust Google and can work around the current defects it is probably worth trying out.