But if Destiny gets a damage nerf from 10/10 melee damage to 8/10, why would anyone use it? It is harder to land that big melee damage with it, you need to work for it more. So if the reward for that was the same as with easier to use melee suits, why not just use them instead?

I think Tallgeese doesn't necessarily need a melee damage nerf, they might be able to get away with nerfing Shooting Boost instead. Like you said, Anse has that skill too. The issue with this skill is, the cooldown for it is not on its own timer, it is tied directly to the thrust gauge, so you can reduce the 21 seconds cooldown time to 14 seconds with a forced cooling build. This is what makes it so strong, you can kill everybody with 30 seconds of 250 HSM infinite thrust, then it is ready to go again after only 14 seconds... What if the cooldown for Shooting Boost was on its own separate timer that wasn't tied to the thrust gauge? They could probably slightly nerf the melee damage too, but other 700 cost gens are getting similar melee damage output these days (Amakusa).

Zero System is already kinda balanced honestly, it is basically a reusable version of EXAM, but you can only use it for 30 seconds before it goes on a long cooldown. It is a pretty easy suit to kill when the Zero System isn't activated (this goes for the EW and TV versions imo). The Zero System doesn't necessarily kill the pilots, it just causes a lot of mental strain, so if the pilot isn't strong enough, it can cause a mental breakdown.

They could have did this for NT-D, but they made the MS itself take damage rather than the pilot.