You picked the right wood for your project. Let it go gray and you'll love it if weathered, smooth, gray wood is what you desire. There is no issue with degradation, finishing South American decking is purely for looks.

You can not seal S. American lumber. The wax they sell for the ends is a gimmick. It does serve some purpose when the wood is really green and is going through the milling and initial drying process. I never seal the cut ends, and have never had an end check appear that wasn't already there. Forget the idea of sealing the Garapa. Any product that seals wood will fail on a S. American wood deck. And the products that work are just dyes with UV inhibitors that color the wood and fade slowly in the sun.
Yes, you can finish it any time in the future if for some reason it makes you happy, but you won't get back the original color once it turns silver. You can clean and brighten the wood, get a little color back, but the sun bleaches everything and it will go silver again quickly.

Good luck and enjoy your new silver deck.

P.S. For the railing, I think glass or horizontal stainless steel cables would look awesome.