I have a very weird idea to balance ekko
Maybe make his stun duration scale with AP. If you don't build AP, his stun will be 1.5s, and if you do you will quickly get it back to its 2.25 duration. One of the big things that is annoying with tank ekko is that he has so much utility without building any AP; hopefully this will tone it down.
EDIT: It is also capped at 2.25s.