You are mistaking Itachi as Orochimaru's goal, as much as Itachi is Deidara's. That's your mistake.
I also feel like you are mixing up Orochimaru's lament about the effort it takes to get Sasuke. He is more complaining about taking down Hiruzen—"at the cost of my arms"—when reposing Kabuto's congratulations on managing to collar up Sasuke.
Like, your comparison only works if Orochimaru had to have Itachi's body specifically, if there were no other options. But the specific Uchiha doesn't matter. His goal is the sharingan, like I keep saying. He does not lose any ounce of desire or will to move towards that.