I have vague memories of perhaps talking about this, but I don’t know when that was lmao, so I’m gonna do it again (if I even did before…).
I think his initial reaction to there being a cure, just generally, would be happiness and relief. This fate, this thing he’d feared for his entire life, this thing that was considered worse than death was curable. Every mage who had been unjustly made Tranquil (read: all of them) could be cured of this horrific fate. It would feel great, and I think he’d want to help spread and implement the use of the cure (something he is uniquely suited to do, of course, and Justice would be 100% into that; righting the greatest wrong ever perpetrated against a mage? Fuck yeah. And Anders could help them adjust after they’re cured as well, since we know that’s tough, and he would be excellent at that). He would also know that if the unthinkable happened to Hawke, if they were a mage, that he could fix it immediately. That would also be immensely comforting, for obvious reasons.
But then there would be a bitter part to it after the immediate relief. Because he would remember Karl, he would realize that he didn’t need to die, that Anders had had the cure inside of him the whole time. Karl could’ve been saved right then and there, but he didn’t know. If he had’ve, that would’ve been a significant thing in his life. I honestly do wonder if Anders would’ve even been interested in Hawke if he’d had Karl around. Karl’s loss had a huge impact on his life, and it would fucking hurt to know it could’ve been avoided all this time.
Of course, he would have Hawke to comfort him. It would help soothe the sting because it was over now, and you can never go back. There was no way he could’ve known. He’d definitely cry over it though, I think. Karl meant a lot to him (I don’t care what he says, he loved him), and the knowledge that he could’ve saved him would definitely cut deep.
But! Anders finding out about the cure for Tranquility, and that he’s uniquely suited to help with that may cause him to realize that he’s immune to it. Which would be… well, I think that would cause him to nearly collapse with relief. The thing he feared most can’t be done to him. The worst thing that could ever happen to him is confirmed to be impossible. And that would be another tearful moment, but those would be of joy rather than sadness, for once.