How about this Thresh change?

Either remove the slow on his E or reduce it hard. But when he hits someone with his Q, the one that got hooked gets a debuff for x seconds that makes Flay slow that target by x (or the knock back is increased). I think the slow could even scale with his Q so that he has to decide between more damage from E or more cc from Q.

He would still be able to catch people and punish you for getting hooked, but he wouldnt have enough cc to kill you after missing his hook anyway (if you get close) and he would be less frustrating to catch/ gank.