@Snowfire, so what is that you actually want to say / do?
I get your thoughts, which is precisely why I want to leave him some time to internalize this discussion and look for his own answers so that we further drift away from the Tywin-Light appearance we have for him now.
[/] Plan Tempest's Forge
-[/] "Do you think I saw the path that led me here when I was but a boy in Braavos? Do you think that the world dances to a hidden drum, that we are all but players, and some of us have a secret way to call changes to the dance?"
Direct attack on his cynicism. We're proposing something insane, and something that also attacks the idea that the Lannisters have any true, enduring power over him. If they had such power, surely they would have used it against Viserys.
-[/] "I came here from my own will, Hound. I fought and killed and won and learnt and from all those things, this city and more came to pass. I never once required fate," the word is not quite a curse, but it is close, "to bring me here."
Attacking the above, layering on a degree of what we've done, and forcing the dissonance around us to push at his world.
-[/] "You can fight, Clegane. You can kill. But you've never won, not the fight that matters to you. And you refuse to let yourself learn what you'll need to do so until you've died trying. You let yourself be wielded by others, and used like the name you took."
This is...well, vicious. Directly hitting the foundations of his worldview whilst they're turning. As close as I'm comfortable with Viserys getting to weaponized psych on someone who isn't an outright enemy.
-[/] "Prophecy is never entirely true, but if what Moonsong said to you appeals? You can choose to make it true, or take a different road. Not being able to see the path until you've walked it is what makes us free. But until you do that, until you choose to step away from this comforting lie that you've made of yourself, then you will just be the man who let his brother do that," point at the burn scars, "to him. And when you die, there will be other chains waiting for you."
More of the above, culminating in slamming an icepick into the heart of his entire world
-[/] "Or you can take that sword, and win with it something that the Mountain will never be able to touch. The choice is yours."
Hope. There is no weapon more lethal and cruel.
-[/] If at any point Sandor attempts to brand Viserys with the same brush as Tywin, or someone like him, riposte with the evidence of his own eyes around him. Sorcerer's Deep is not a city of Westeros. No matter how hard Sandor might try to deceive himself, the proof is right there in front of his eyes. He can run from that if he likes, but he can't hide from it now.
I'm not 100% certain that this will work. I don't know Sandor all that well. But I know his mind, or at least one like it. This could probably use some touch-ups, but at the end of the day,
@Azel, if you have dissonance and want to make it stronger, you use it. Sandor isn't someone to just...allow to sit there and stew. He'll just go back to what's familiar. That's what he does. A proper counselling environment would give him the safety to explore a world where that wasn't needed. I don't care enough to try to give him that, and I don't think it would work, anyway.