There are two different issues that can cause pitch changes with speed changes. One I'll call "speed sensitivity" the other "power sensitivity." If the thrust line is wrong, there will be an immediate pitch change when power is applied or taken away, most obvious in a sudden throttle change. The actual application of power is causing the pitch change because the thrust from the engine is pointed in the wrong direction in relation to the drag of the airplane. That's power sensitivity. Some planes though by virtue of their design can't help but gain and lose altitude with speed changes due to their lift characteristics. The worst offenders here are the planes that are designed to make lots of lift at low speeds like trainers and Cubs. The issue in that case has nothing to do with the thrust line and everything to do with the CG and wing and tail incidences along with the wing airfoil shape. Identifying which issue a plane has is the first step in actually fixing it instead of applying a bandaid.

The problem with using an elevator mix with the throttle to fix either of these problems is that changes in the plane's speed lag behind changes in throttle. So if there is a thrust line problem, the plane will react to the power first, and then as it builds speed it will start to even out (not completely, but at least some). So fixing it with a mix produces a plane that responds properly to throttle changes but flies out of trim once airspeed catches up. If it's the speed sensitive problem, you'll actually get the opposite effect- the plane will fly in trim once airspeed catches up but will pitch with throttle changes. Programming in a mix is a fix for the very rare plane that is flown at a constant throttle setting for most of the flight and has an aerodynamic design that simply cannot be trimmed to fly straight at all throttle settings. The OP will be happier with his plane if he ferrets out the aerodynamic problem that's causing his trim changes and fixes it rather than applying an electronic fix to cover it up.