1. Assign Toggle thrust reversers to a key. Push the key after landing and advance the power levers to maximize reverse thrust. I find this to be the best solution.
2. I use the CH Throttle Quadrant for my power, prop and condition levers. With this hardware, there is a check box in the X-Plane 11 joystick assignment for the throttles, "Throttles are CH Products Throttle Quadrant." If I check this without the reverse thrust key assignment, reverse thrust is automatically engaged if I pull the power levers back beyond the detente built into these throttle levers. This method is more realistic but it is difficult to pull the power back to the minimum for landing without accidentally engaging reverse thrust!
3. There is a lua script, natural_reversers.lua that enables fine tuning a lever, even if not a CH Throttle. I had the same problem with his as with #2 though.
It's great that you are studying as much as possible about the Avanti. I do this with all the aircraft I fly and find it half the fun of flying in the simulator. I downloaded the full pilot owners handbook from here. It costs $25 for an annual subscription but I think well worth it, even for one manual. Having said that, I managed to download all my other handbooks for free.
I hope this helps!
Kor