B-HKD
You are over thinking this.
Fly-by-wire aircraft are not B737's with a computer moving the control surfaces directly. Unfortunately with the Boeing control wheel it gives this impression.
The control wheel or the side stick in the airbus could be seen as "aircraft desired state requests" i.e. tell the aircraft what you want and the FBW will manipulate the surfaces iaw the designers algorithm to achieve this state.
The FCTM.
The primary difference from a conventional airplane is in the pitch axis. A control column input commands a pitch maneuver instead of commanding a surface to move, so anything else that tries to change the pitch attitude or flight path will be countered by the flight control system.
The FCOM.
When pilot trim inputs are made, the PFCs automatically move the elevators to achieve the trim change, then move the stabilizer to streamline the elevator.
In reality just fly it like an aeroplane trim out the forces as required.
You are not trimming the stabiliser but the the elevator and the stabiliser then moves to streamline the system.