you shouldn’t use RC trim for that, use SERVO1_TRIM and SERVO2_TRIM.
The RC1_TRIM should be set according to your transmitter, and has nothing to do with the servo outputs. The SERVO1_TRIM and SERVO2_TRIM are what control trim of the elevons.
Alternatively you could use SERVO_AUTO_TRIM=1 and it will learn the correct trim while flying.