Try treating your servos as "tilt" servos (like a tilt-wing, they rotate when you do the transition). In the mixer file this would look something like this:
O: 10000 10000 0 -10000 10000
S: 1 4 0 20000 10000 -10000 10000 <-----giving it an offset will increase the servo travel closer to 90deg
Next, you probably don't won't the servos rotating during the transition, only when it finishes and goes into FW mode. To do this, go into QGroundControl and find VT_tilt_trans : "position of tilt servo during transition". Set it to 0, that way it won't move during Phase 1 of the transition, only retracting during Phase 2 (when you should be ready for FW flight). You also get control over how long you want the retraction to be, and at what speed etc.
Hmmm however you probably want to use "Standard" configuration, in which case the "tiltrotor" parameters may not be available, and the tilt control actuator might not work. In that case you might be able to try an aux command that's connected to the transition switch in someway, but I'm really not sure. Interested to hear if someone else has an idea