Hi,
I've been a playing zed for a long time and I am a diamond zed 1-trick. I can definitely understand where you are coming from. Zed and most champions in league these days have high burst when ahead. Especially this season with dark harvest where the issues are exasperated. I understand that you think he is broken but when he is ahead (levels, gold, items), he should be able to 100-0 squishy champions who do not have armor. That is because that is the job of assassins. Even champions who do not fall into that category have that capability. Think Jayce who eq's for half health followed up by him jumping in and finishing with an auto or two, Annie or ahri who can 100-0 if they land their cc when ahead. Almost every champion in the game that isn't a tank or support has that level of power when ahead.
Further, nerfing ANY champion's damage by 40% is a MASSIVE nerf that would make them unviable with the except of their kits being super overloaded with cc, utility.
Zed's weaknesses are that he is melee which means he has a tough time in lane should the enemy laner be competent and understand the matchup as well as the fact that Zed's poke rotation is on a 22 second CD that does +- about 100 dmg if he whiffs even 1 q. As an assassin, his job is to punish players who are out of position which is why I believe you think he is OP. In low elo especially, people are almost always out of position which makes him either an low elo pubstomper or useless. Simply looking at winrates, he isnt that strong at any elo.