Hi Michel, welcome to permies
What an amazing place! I second Tyler's question: are you confident that it's infertile? I have no idea what that area *should* look like, but it sounds like there's a lot of biodiversity, which is a really good sign.
I've got lots of questions...
Is any of the grassland original savannah, or is it all ex-forest? Any idea when it was cleared?
Should I assume the rainfall's quite high? What are ther rainy seasons? Does the spring always flow?
What's the drainage like?
Is it windy?
I don't understand your soil test, maybe a Portuguese speaker can help.
Is there a result for organic matter on there?
From what I know of soils, if the area is naturally acidic, trying to increase the ph to any major degree would be an endless, unwinnable battle.
I grew up in an area with very acidic soil. It has very high rainfall, and major nutrient leaching on the cleared land. Sound familar?
I ask about drainage as waterlogged, anaerobic soil is often especially acidic.
While I'd be wary of saying "do this and your ph will increase"; adding carbon and other organic matter to the soil, as well as earthworks to drain and hold water, would probably help.
Aside from that stuff, I'd recommend researching what trees/plants/vines do well locally, since they'll be adapted to your conditions.