RE: Trim for level flight throttle settings
If the airplane is built straight.... the only trim that should need adjustment with throttle is elevator. We don't fly straingt lines long enough for a minor aileron or rudder deviation with speed to be worth adjusting the trim.
Elevator trim can also need adjustment with fuel burn-off... due to the CG slowly moving rearward.
The need for retrimming with speed change is why I prefer the old analog trim sliders over the modern computer radio pushbuttons. You can learn where the trim sliders need to be for different flight conditions an quickly make the adjustments. I'm constantly retrimming any airplane as I change what I am doing with it.
Full scale aircraft... the pilot retrims for what he is doing with the aircraft. they learn that the plane neds X turns (or X "clicks") of the trim to go from 60 mph putting around to 120 mph cruise.