It would need to be a staggered thing. Even maybe level 2/3 would be needed during some of the stages. The stages will need to be long enough and/or subsidized enough to let every road user get appropriate vehicles under predefined timelines.

Obviously I won't pretend I worked out every single details, but I just don't think leaving it up to Elon&Co to figure it out while gambling with people's lives is the right way to go.

It sounds like you're from the US so I do understand why you'd think countries making companies work together towards something like this is impossible. It might be, there. It would be a colossal project tbh, but I stick by my opinion that it needs to be a transition supervised by regulatory bodies and not just the wild west with every company doing different things.

The whole system will 100% have to be unified to support full self driving.