The gatekeeper warned him against using the dark energy of the island but also said it might be possible if he gets older. I do think though that you have to remove an eye to use it. I do not expect any shortcut.
It probably needs a lot of hate as well. There are implications for using it.
Oh he definitely needs to be careful. But having that power channel through a dark foci rather than himself would have to make a huge difference.
As for losing an eye to use it, maybe. My first thought, if that's the case, would be to have Alfred use it. But I'm not sure how ready Harry is to go changing Alfred's programming.
I'm not sure we've met an ally yet that's big enough, metaphysically, that can wield it. Maybe Vadderung, but I too think he'll be dead by the time they want to use it.
It probably requires rate and hate top wield it, and I'm not sure who fits that bill.
Unless we get gross, and Harry can literally release parts of inmates, and he puts Ethniu's head in a crystal jar whenever he needs to pop the eye in and use it. But that seems unlikely for numerous reasons.
Unless he meets a new ally in the wrestling book that's an immortal the he trusts with it, he might just have to find another way to use it.