For my subtle zooms, I keep my keyframes between 10-20 frames apart from each other, like this and this. Also, I keep my zooms going in the same direction; if it zooms in from Frame 1 to 10, I keep it zooming in until the end of the media.
For my dramatic zooms, I zoom in 5 frames then begin to zoom out, like so. Unlike my subtle zooms, my zooms change direction at Frame 5. Out to in, in to out.
My suggestion is to test out with different keyframe numbers and how far you zoom in; I stick with 5s and 10s because it’s easier for me to remember those numbers.