Your question is a bit of a ‘how long is a piece of string?’ sort of question, there are just too many variables. Every TBI is different, some people can have what is considered to be a minimal injury but the effects can be devastating, others can have a major injury but have minimal effects. So trying to gauge your personal situation is impossible.
If you do decide to go travelling you need to be asking some serious ‘What if’ questions. In your travels what if you have another injury? Can you get the care you would need in some of these countries? I doubt you’d be able to get comprehensive travel insurance, especially to cover your head (Most exclude pre existing conditions). If you had health issues would you have the funds to medically evacuate from anywhere on your route? Medivac is REALLY expensive and I’d question whether such a service would even be available in some South American countries.
A very common occurrence with TBI is over estimating our new abilities (Well, it was with me anyway). I thought I could just do as I had before. OHH BOY, was I wrong. I tried to push myself to reach my goals. BAD IDEA. With any TBI, recovery is a slowly, slowly process. My last neurosurgical procedure was back in '13 and I’m still battling with some deficits. Alaska to Argentina is a huge ask. For a starter, post recovery, try some smaller trips, see how that goes before you try such an arduous journey.
There are many, many things to be taken into consideration here and you’d be well advised to sit down and think of all of the worst case scenarios before even going near such a trip IMO.