I actually helped the Khmer ally right away and took the Tambralinga base on the Malay peninsula. Massed some ele archers and once I had 15 of them, with about 2 trebs I went for a landing in the middle and trebbed from the center of the map to the west.
I never bothered making *any* navy as you can command an infinitely respawning navy.
Ele archer + Urumi eventually became my comp. Once you have around 40-50 ele archers you are just unstoppable. The key is trebbing the castles only. I left the entire base intact every time and let them send their units into a ele archer meatgrinder that safeguarded the trebs. Once you have the west, the push into the eastern castles is somewhat slower due to the spam, but your comp is fantastic. I started to make some siege eles as well.
Never bothered with the easternmost base that has bbc on it.
TL;DR: don’t make ships expect transports. Help Suryavarman first and get a safe base off off Sumatra. Make ele archers almost exclusively. Only destroy their castles