As far as we know, Toland doesn’t want the mantle for himself. His original motivation to help the First Crota Fireteam was curiosity:
I am so terribly curious to know.
Additionally, Toland is (in some small way) helping us in TTK:
Are you following this? Would it help if I etched a few notes on the margins? I didn’t shuck my mortal form and smuggle this nightmare arcana back to the waking world for the benefit of that masked hypocrite’s drooling loyal orthodox.
And he’s angry at us post-King’s Fall, when we leave the throne vacant:
You fools! You disastrous, bumbling squanderers! It’s not right! Who now shall be First Navigator, Lord of Shapes, harrowed god, Taken King? Not you! You might have been Kings and Queens of the Deep! But you have toppled Oryx and you have not replaced him!
If he wanted the mantle, this would be his free way in. But instead he wants us to take the throne, instead of simply telling us to go and assuming the mantle himself.
Given that Dredgen Yor is the remaining subject here, I’d say it’s impossible to remain “who you are” while also utilizing the power of the Darkness. Yor didn’t exactly keep his marbles:
[u.2:2.8] Sit down.
[u.2:3.0] Sit. Down.
[u.2:3.1] Your mouth just got your friends dead.
[u.2:3.2] This is what happens when you bore me. And right now…
[u.2:3.3] I’m so very bored.
[u.2:6.6] Open your eyes.
While the Shadows of Yor are attempting to get Yor’s power without “fall[ing] completely into despair” in the same way he did, who knows how successful their endeavor will end up being.
The other example of the Deep changing one is, well, Oryx himself. Aurash is very, very different than Oryx, and that change was largely a result of the Darkness. Quria’s simulation of Oryx didn’t recognize her future self at all:
From within the Hydra-hull, Quria’s tiny not-Oryx speaks. “What are you?” it says. It’s manifesting terror and awe.
Taking up the Deep seems to, at some fundamental level, change whoever mantles it.