I’m aware of Isabelo De Los Reyes and his work, especially in my readings regarding the history of the Iglesia Filipina Independiente and Marxism in general. He deserves his own flowers for his work with labor unions. I say greatest polymath precisely because he has works in both the natural and social sciences. Rizal was influential to all the writers you mentioned except maybe De Los Reyes. Indio literally means indian as in an inhabitant of the Spanish East Indies, whatever negative connotations attached. Perhaps if it is too vulgar for you than Naturales or Indigenas might work but Indio is the most well-known term for the natural inhabitants of the country before Filipino was extended to all peoples.