Have you considered yet another old myth about a sunken Tamil homeland known as "Kumari Kandam"?
Some people place it in the submerged land that is now between India and Sri Lanka. But legend has it that it was the size of a small continent, which opens the idea that it could well be in Sundaland.
The geneticist Stephen Oppenheimer spoke about the Senoi tribal people from Malaysia have skull shapes similar to the Tamils. It is unclear if the Senoi's ever ventured down to Indonesia, but it is possible given that their presence in Malaysia dates back to the time when Sundaland existed.
On the other hand, the Negrito people have had a presence all over SE Asia and even Australia. And these were the very first people in SE Asia, that we know of.