QGCMissionNavWaypoint - defaults to 0deg takeoff pitch.

It's dangerous.
I would set default to 10degree , if not else, so just to make it more "visible" during editing, and harder to forget.

Could you please point me to where the default is stored/set ? (or do it) - I've tried to modify QDoubleSpinbox.value but it is populated by 0 anyway.

Search for the QGCMissionNavWaypoint entry point and inspect where its QDoubleSpinBox value is initialized or populated. Confirm the takeoff-pitch default is 0 degrees, then update it to 10 degrees and verify the value shown during waypoint editing.

Written by the indexing model from the issue text.