Be very aware that if you move your cg back 3 cm, the plane will be MUCH more sensitive to elevator inputs Maybe do it in two or even 3 steps.

Nothing wrong with an 12 x 4 yes it is a 3D prop but it will fly MUCH slower on a landing approach with the same RPM which is what the OP wants.

Might be overrevving a bit in a dive but thats what a throttle is for.