Let's start at the basics. Tentacle placements. I very much recommend watching this video made by DirtyMobs
Some more facts that you should keep in mind -
Your tentacle damage becomes lower with every tentacle that hits, so with objectives you should space them apart instead of bursting.
Your Q is also very very handy to poke with, not only your E!
Your ranged w trades with grasp are very strong early game. Don't forget to use them.

I personally don't like maxing E. I think that killing the spirit provides more pressure. When I'm playing Illaoi I want to act like a pressure cooker and just keep the enemy under constant pressure. Additionaly, killing the spirit spawns more tentacles closer together which makes our pressure higher. In addition, the Q max helps us survive in situations where we miss our E. If we miss our E and get all in'd by a ghosting Darius that is an awful situation. With Q max we're not putting all of our eggs in 1 basket and we can still potentially kill Darius despite missing E. All in all, if you can consistently kill the spirit and clear the waves fast than it's your choice.