“The real Shaalah is if psychological stress counts as Tzaar. Did you see anyone discussing this question?”
Nobody agrees that this is the question. This whole psychological stress was fabricated in the first place. Even if you could prove that every Chabad chosid felt psychological pain by trying to sleep in the sukkah, that would only be because he was taught to feel that way. Nobody else is going to automatically feel mental stress when trying to perform a mitzvah. If your rebbeim have convinced you that you should then that’s terrible.
For the point about retroactively justifying minhagim, yeah I kind of have to give you that. You brought out examples from Chassidish minhagim, but everyone has this kind of thing. Eg. the fact that Litvish yeshivas don’t have mikvahs b’shittah, the fact that litvish bochrim are clean-shaven b’shittah, the fact that litvish bochrim could go years without saying korbanos b’shittah, etc.