The only Indonesian island I know of that remains mostly Hindu is Bali. There are bits and pieces all over, and if I’m not mistaken, Hinduism is expanding somewhat, Islam being pretty notional in many parts.
Bali and the next island east-ish (lombakan?) are the only ones I’ve been to. They told me there much the same as what you’re telling me...that Islam is spreading despite the dislike of every non muzzie.