In the first post was stated start feb-march.
The excluded to start for ex in Canada.
Later you say that you can start whenever.
If you start in summer time in (for us in the north)
Then NA have many pros.
You start getting used to some things.
You start in a country (Canada or US) where you know the language.
Where the culture is similar to Europe.
The infrastructure is good.
Shopping malls everywhere. If some equipment is missing. It is just to stop and buy.
No border crossing (Except Canada-USA. Easy one)
You can travel the whole of USA, without border crossing.
And get used to the life on the road.
You take the Spanish and the border crossing in Central America as a later step. And the climate,culture and infrastructure in SA later as next step.
This why I think more people ride from Alaska to Ushuaia than opposite.
Where in NA ? Your choice. I would start in Canada. And after Canada go South. Some supporters of Alaska will disagree.
To ride Dalton Highway to tick that box is not my taste. But will be important to others.
Ride around USA towards Mexico.
Inspiration:
https://www.loispryce.com/books/lois-on-the-loose/