It might be easier to collaborate with an established producer of a specific vehicle and create a partially custom vehicle from the same "car platform".
The “add on” thing belongs in he list of tarpit ideas. Watching Cruise and Ghost Automation flail about for years vaporizing cash trapped on this idea was like watching a comedy.
Waymo got away with what they did b/c they stayed focused and didn't grow too big, but they could only do that because they had Google to finance them.
Traffic laws are a hell of a lot more lax than the situation around on the job injuries.