When I say "no joy" I don't mean "no pleasure in doing anything, ever" but rather I mean, "not happy in life," "no great pleasure or joy". I don't think either character should be come a characature.
I don't think we want to go quite that far with Maedhros, especially if we do with Amros. A lot of Maedhros' likability is going to rest on his love for his brothers, friendship with Fingon, and assistance in rearing Elrond and Elros, after all, and I think he'll need to smile and laugh with them, even if only occasionally and perhaps with a constant twinge of melancholy, for that to work. But having an almost unholy glee at the slaughter of orcs? Sure.
I don't think that really works. He's going to be
completely wrecked and in the Silm, he never emotionally recovers. Again, look at Gwindor, who suffered far less. He never laughed again, and only rarely smiled when he saw Finduilas (before she started loving Turin...). His life is ruined, as he says to her later. It's not like he has a death wish, though, as he reproaches Turin while dying.
I can imagine Maedhros can smile once in a while, rarely. He laughs at Thingol, but out of racist contempt. Actually laughing for mirth, I can't see. But I could imagine raising foster-children as the first thing in a long time that he actually enjoys. That would be sweet.
And we can go further than "unable to find true joy" with Amros, I think -- we could go to "doesn't care to find joy, at all". He's there, he's existing, but he's not living. He's got a job to do, an Oath he resents and hates, keeping him from going home and reuniting with his brother -- if the Oath didn't make that impossible. Maybe, in his darkest moments, when he fears his twin is lost in the Darkness, of breaking the Oath and casting his own soul into that dark, as the only possible way of reunion? We could depict his resolution upon attacking the Havens as a death wish.
Oh great, now you've got me envisioning him like Crazy!Denethor. "Burn! We will all burn!" That could be really, really disturbing. (Disturbing is good -- every Kinslaying should be.)
I don't want him to behave that way constantly for 540 years, though. "In his darkest moments" is enough. He's got to be more functional than Crazy!Denethor most of the time. Mental illness is fortunately only sometimes that dramatic (and that applies to what I said about Maedhros, too).
But we can make up whatever "torment" we want for the Oath near the end. What exactly it does to torment them, Tolkien never said. I was imagining something like the Ring tormenting Frodo -- the constant intrusive thoughts of desire for the Silmaril, until the light of the Sun and Moon seems sick and dreary. What if that's combined with visions of being cast into the Everlasting Darkness... and of your twin who might be there
even now? And Amros' response is to dredge up the worst of his misery at that moment when he heard Amrod screaming in Lammoth. Your idea that he actually desires to go to the Darkness is...
really, really disturbing, like perhaps darker than I'm comfortable with. It would be a very contrastive reason for him to try to break the Oath, compared to Maedhros and Maglor who do so out of remorse. But so, so dark. :/
Until that time comes up after the Second Kinslaying, though, Amros needn't be mired in nonstop depression every year.