Remember that the point of up thrust and down thrust angles, are to fix problems when power is applied or remove.

In effect if you pulled OFF from full throttle, and the plane were to continue moving forward at the same speed, it's attitude should NEVER change assuming no control inputs. ( Conversely pushing full throttle, when at normal flight speeds, should have almost the same result( s ). )

This is what Up-Thrust or Down-Thrust fixes if there is a change.

In reality the plane starts to slow down, so typically the plane holds the same attitude briefly, then begins to start changing attitude as speed decreases.

With proper up or down thrust your plane may still dive all too quickly, but not more or less so than when at throttle.

An excessive tendancy to dive or climb must be seperately addressed.