Potential Zed Rework Idea?

So first off, let's be honest; as late game rolls around, Zed's ult damage becomes pretty BS. Especially with the introduction of Duskblade and how most Zeds are building full AD and armorpen now, all he has to do is hit a squishy with E-auto, a hydra active, and an ignite, before he swaps back to safety, and the damage on Death Mark will be enough to finish the target off. That's not exactly fair, as Zed's kill combo is balanced around the fact that he has to hit his Q, a very narrow, easy to dodge skillshot. Now that QSS cannot remove his ult anymore, he has a pretty unfair amount of reliability and safety when diving ADCs.

This is why I think that we should really just overhaul his ultimate a little bit, and here my idea. I don't exactly know what numbers will make him balanced, but here's the basic mechanic. So, instead of taking % of the damage dealt during the 3 seconds of his ultimate after already taking a beating from Zed, the target of Death Mark now becomes immune to Zed's physical and magical damage(so his ignite can still work on them) for the duration of the ultimate. However, after the 3 seconds are over and the mark pops, it instead deals damage based on how many times the target was hit by Zed. Let's say the target took all 3 shurikens, 3 E's, and an auto. That's seven hits, seven stacks, and the mark will deal the deal 7 times whatever damage would be balanced. Ideally, at five stacks, the mark would deal enough damage to 100-0 squishies with 2000 HP. After 7-8, it would start to drop bruisers to a dangerously low level where Zed can swap back and execute his target with his passive. Now, maybe counting Zed's autos as stacks are a bit too much, but I genuinely don't think that a Zed would be allowed to chase after a high-priority target during teamfighs and relentlessly autoattack ADCs. If he is, then he kinda deserves every single stack that he gets off, because the enemy team is obviously not focusing him correctly. I also don't know whether this damage should be AD, AP, or true. I just know that this not only makes Zed have to actually hit his spells late game, but also eliminates the "feast and famine" style that Riot hates so much, since this version of Death Mark deals damage based strictly on how many spells you hit, and not how high your stats are.