You don‘t use trim to maintain a pitch (or vs) but you use your controls. That‘s what they are for.
Trim is used to release pressure (force) from your controls. If you constantly have to push then trim forward until the pressure is zero.
In level flight with small aircraft you use the throttle for very fine adjustments as the airplane will want to hold its speed. But first you trim it properly. If you have a rudder trim you‘ll learn the lesson quickly: every power change requires you to adjust the rudder trim. If you don‘t have it you either now the power setting where you can relieve the pedals or you will have to put pressure on one leg.
Throttle for descent and pitch for speed is something else. It‘s a basic understanding in single engine airplanes because altitude is speed and speed is life. If you fly without engine you regulate your speed with your nose only and you use this concept on light aircraft for safety reasons. But practically pitch and throttle go hand in hand. In heavier and sluggisher aircraft you‘ll use throttle/thrust for speed.