Unfortunately it is not this simple.
This depends mainly on your weight, the air temperature and air pressure (look up density altitude).
For the best performance you should fly at 50 kt IAS (Vy), any faster or slower airspeed will require more power.
That’s for flying… Hovering takes much more power.
It is actually quite realistic, that you can’t take off at a 9000 ft airport.
That is why it is so important to understand all the charts in the performence section of the flight manual.
For taking of you would need at least in ground effect hover performance (excluding a running takeoff, where you slide on your skids until you get fast enough to take off).
So what will help to take off at high altitude?
A strong headwind will be your best friend, reduce weight and have a lot of space to build up speed while you are still in ground effect (try to think of it like a takeoff run in a fixed wing aircraft). Build up speed until you hit 50 kt and only then increase your altitude.
The Cabri is not really made for taking off and landing at 9000 ft, if you want lots of power at high altitude you need a different helicopter.
And yes, you do not lean the mixture in the Cabri, in fact I have about 120 hours in the real Cabri, so I can confirm this.