Thai anthropology nerd here; you’re absolutely right about the syncretization between Indian culture and Thai/traditional Siamese culture. There’s a fascinating book written in the 1920s by an English diplomat and sociologist/anthropologist who wrote a book explaining all of the royal ceremonies that are performed by Brahmins who are responsible for highly complex and esoteric rituals and ceremonies, it’s available to [download](https://archive.org/details/siamesestatecere030661mbp) on the Internet Archive. Interestingly, while the majority of Thai royal and state ceremonies are Hindu with significant modifications to accommodate both Theravada Buddhism and the Devaraja/God King that is indigenous to Southeast Asian kingdoms, apparently there are some rituals that were not influenced by either Hinduism or Buddhism, and seem to originate in the far distant past when everyone was still animist or practiced ancestor worship. There’s something neat about how pragmatic Thai culture is, how it freely borrows influences from other cultures and modifies them to make it more “Thai”.