"dead throttle input"
Is that what you meant before? That would explain your near perfect plug reading. Do not lean further, but increase prop load.

In a walbro carb that lack of response in the last throttle travel is normal. The effective carb throat opening does not increase in the last 20% of throttle travel. You might as well stop servo travel before the throttle arm hits the stops.
18x5 is too small again. 18x6 would be the smallest I recommend for the engine, for static rpm just in excess of 9000.
That is only for 3D.
For Imac type aerobatic flight, 18x7 or 18x10 or 17x8-10 is way better.