When I started getting into tec I wanted to make the leap to trimix early, but I was persuaded to wait a while. Introducing helium into the mix throws in new wrinkles. Some of the gurus on the board can pick me up where I slip, but from recollection the key "new" things were:
- He is absorbed and off-gases much faster; conceptually you approach deco in a new way because He is much less forgiving.
- Risk of HPNS
- The nitrogen "step" - not jumping to a deco gas which has a significantly higher partial pressure of nitrogen (can't recall the reason for this, but I remember that it was a bad idea)
However, my understanding is also a lot of what you say is true - you also build further on skills you already have. It is just that before where you might have used one or two stage bottles, now you have three or four (and they are bigger). And if something goes wrong, you are (literally) deeper in the *$&%#.
I am hoping to do a trimix class next year. Until then I will just enjoy the narcosis buzz and/or fret about the fact that my fins are eating my legs.