Went with street view to see your route. see a lot of tarmac and big wide roads (not all of them). are you sure you want to join motorised traffic?
Using small roads and gravel roads to me is the essence of bike touring, be alone on the road and enjoy nature. Why shouldn't it be safe?
Usually, I always take small roads but this time, it is Mexico, and several government mention the danger of being there. And those small isolated roads seem a bit risky for my security (at least in Tamaulipas state)
Currently, I have two overlapping routes planned. So I can choose according to the feeling of the road. Also, I don't want to get stuck in the middle of the night in the middle of nowhere If I'm deciding not to bring my camping gear. (still not decided yet!). All my gear are in Texas right now waiting for me, so I have exactly one week to decide!