If I were God? Hmmm.
First, dealing with multiple tribes is troublesome, as you are juggling competing interests. I'd make tall mountains splitting the island into zones while I individually worked on them.
I would make use of complex subliminal dream technology. Rather than telling them what to do or what to think, I'd send VR type lucid dream scenarios, each of which demonstrated the consequences of their actions. If they behaved like brutes in the dream, the land would dry up. If they worked hard, and worked together, the land would be super-fertile. Basically, teach them the Nash game theory.
Next, I would work on implanting a beauty ideal that was common to all islanders, and then selectively breed the islanders based on preference to evolve them physically from big dumb brutes to metrosexual men and equally slim women (a sort of Asian unisex ideal). I'd also do genetic manipulation to make some of the tribes have an excess of females, and some have an excess of males. I'd implant another idea, taking advantage of their exploitative nature on how "exotic" these foreign men and women were, and have them slowly mix (any attempt of violence or rape, however, gets rewarded by lightning from above) until enough of them have family alliances, and violence becomes a costly mess.
I'd teach how to perform trade for uncommon goods. And slowly, I'd introduce concepts of medicine and agriculture, leaving them out of the loop on advanced warfare. They might still learn, but guiding them towards this direction, would help them live in peace.
The unfortunate thing is this is a world without much free will, so I'd probably get bored eventually and reveal myself, invalidating all my progress.
