Unfortunately, during flight, there is no way to prevent the camera from controlling the flying mount's pitch attitude, besides a toggle option that only applies to auto-move. While it gives full control over the mount's flight direction, this makes it hard to fly while looking at the terrain below, unless one is trying to land.

The "use standard camera for auto-fly" option, when unset, does prevent the camera from affecting the attitude after auto-fly is enabled; however, it will kept the pitch attitude the mount previously had at the point it is enabled.

It would be nice to have a "constant altitude" flight controls mode. The left stick would behave like in the Legacy control mode in the ground, moving the character relative to the camera along a plane, but the right stick would not affect the pitch attitude (and thus the altitude). The mount would fly at constant altitude relative to the plane of the map, and the Y and B buttons could then be used to change this altitude.