Static RPM is way too low
I have had this before. It overloads the engine(lugging). The EGTs will also e low.
That loss of power is the piston grabbing the cylinder wall. Reducing throttle makes it go away because you are reducing cylinder pressure and temp.
Get the static up around 6400-6500 and go from there....Find the balance.
Too much pitch = engine lugging, high cylinder pressure with low EGT
Too little pitch = lower cylinder pressure and higher EGT
One other thing you didn't mention in your list of things you have checked was the float bowl vents. Rotaxes are very fussy about how they are set up.