My decks are of Ipe, sometimes called Brazilian Walnut, and my results are similar to yours. I don't believe there is any penetrating product that will last long outdoors.
I use a Penofin product made specifically for exotic hardwoods like Ipe. I apply it about every other year, and after every application, the decks look great, but slowly fade to gray by the next season. The exception is a porch that is sheltered from sun and rain, and it looks good for years.
Any kind of varnish would be a mistake IMO.