Hi,
GPSAssist is not needed above a GPS mode. Anyways GPSAssist is only supported by Multirotors.
Seems you changed many Thrust parameters/range inside AltitudeHold/GroundPathFollower/VTOLPathFollower settings.
The only used by Wing frame is FixedWingPathFollowerSettings where you reduced the thrust between 0.1 and 0.3
You should keep default settings at first, and try to get a decent working Attitude mode stabilization.
Before using Attitude you should try Rate, check if you got oscillations and reduce PID if needed (inner loop)
For a more reactive attitude mode you should increase:
Stabilization > Settings Bank1 > Advanced > Attitude mode response
Stabilization > Settings Bank1 > Advanced > Attitude (Outer Loop)
https://librepilot.atlassian.net/wiki/display/LPDOC/Attitude+Setup#AttitudeSetup-AdvancedtabFor better turns you can adjust the Vehicle > Roll diff to reduce adverse Yaw
https://en.wikipedia.org/wiki/Adverse_yaw#Differential_aileron_deflectionWhile flying in Attitude and you increase Throttle the wing should climb a little : Adjust Attitude > Pitch rotation.
After you got a good Attitude stabilization you can try RTB.
Be sure the Mag still without Red alarm when you apply power.