having trouble crossing up crouching opponents (non-meaty) and landing combos. should the dive kick be done at the top apex of the jump?
Best spot to hit crouching opponents is at the bottom of their neck,around their shoulders. Unfortunately not all characters have the same width or height when crouching so it is something you have to try for yourself and figure out the right angle and timing. On smaller characters FJ LK Dive is fine, on the bigger characters if you want to do a crossup on a crouching character with a LK DIve you have to do the LK Dive from the FJ later. This result in it being much easier to react to and they’ll stand, which is not what you want. NJ HK Dive is faster and easier to get the proper angle with. You can still use LK DIve agaisnt bigger character, i mostly do NJ LK Dive to stay in front and catch crouchtechs.
It is mostly about the angle that allows you to combo so try it out, not every character is the same so there is no one timing for all.