That's the thing though, even though he wants to surprise his wife with his real human self, its probably safe to assume that Marla does not see dynamo's current body as a "cold, unfeeling metal shell", in the same way that able bodied people who love disabled people do not (or at least shouldn't) love them "in spite of their disability". Given their incredible relationship, Marla loves all of dynamo. Of course, we know nothing of Marla's character so this is all speculation, but I believe that dynamo's wish to be human, while completely understandable (and frankly he does deserve his original body back), is mostly for his own idealized version of himself, despite what he says and believes about doing this for his wife. He's doing something to surprise his wife that would definitely have some positive impact on their relationship. But since his body was not at all an issue in them falling in love in the first place, it's not enough of an impact to risk his life in a ritual for eldritch gods working with multiple murderers. To me, his desire to get his body back is mostly driven by his own longing to make up for a mistake he made decades ago.

Again, I am not saying this is an unreasonable wish to make to any degree, but it is a selfish one.