After some searching, I've discovered that there's nothing I can do from my app to open one of the major map apps and start turn by turn navigation immediately. (If you know, please tell me)
In Google Maps, Apple Maps, and Waze, I can deep link into the app and have the route shown to me (usually by setting "saddr" and "daddr"), but human intervention is still required to actually start the navigation. All of these are one click away, but then the actually "Route, start, or go" button is quite small and hard to hit on a phone while you're actually in a car with your phone on a dock.
I'm just curious why this is, I can't figure it out. The only hunch I have is that they have to open a more persistent connection with the device and they want to avoid DoS attacks or something like that. It seems like an easy flag to put in their API, and I've seen various other questions trying to do what I'm trying to do.
And even if all that is true, they can still probably have a different flag where navigation is one click away with a button that covers half the screen, much better than a button barely the size of my fingertip.
Hopefully someone has some expertise on this so I don't go yelling at engineers working on the map apps.
The answer is simply that none of the major map app developers have allowed this. There is no technical reason why iOS apps couldn't do this; they just don't. The decision is probably because they want to require user interaction before starting directions, as most apps allow the user to edit parameters or choose one of several routes before navigation starts.