Interesting to know about Bengali connection too. You may know already that Tamils, Bengalis, Assamese, Thais, Burmese, Sri Lankans all have the same new year (testament to the older links forged in the formative eras around Bay of Bengal...i.e adoption/continued use of the older solar Hindu-Buddhist calendar).
Well before the Cholas btw, were the Pallavas who were probably instrumental in spread of Brahmi/Grantha script to South East Asia by trade:
http://skyknowledge.com/pallava.htm
This is also probably how Hinduism spread to the area first (adopted by the elites/rulers of the South East Asian Kingdoms especially). Then came the major spread of Buddhism which further strengthened the bonds to the script system. Then came the Cholas after that (who had made alliances with the Khmers and opened a front in honour of that alliance against the indonesian/malay Sri Vijaya empire who were fighting a sustained conflict for supremacy with the Khmer empire...especially given Sri Vijaya had increasing control of the trade routes between India and IndoChina/China). This led over time to decline of Sri Vijaya (Cholas later would ally them as well, but by that point Cholas were on the decline too). Eventually subsequent Majapahit kingdom would be the last major Indic/Dharmic style empire in Indonesia
@Indos @Marine Rouge @trishna_amṛta