I understand that Nezuko gets stronger from just sleeping but the entire time Tanjiro is training to master his breathing alongside Zenitsu and Inosuke she just sleeps. Would it not have made more sense to train her to breathe as well and she could sleep during travel and daytime and do breathing training at night? Or is this not done due to just plot, not constraints? Are demons even able to learn breathing techinques?
Nezuko sleeps not because she has nothing else to do but because in such a way she restores her strength instead of eating flesh. She slept through Tanjiro's training year, while (probably, not shown really) he tried to wake her up.
From the slayers' point of view, they most likely have no idea how to train demons because they have a different physiology. (Upper Moon One seem to have mastered the moon breath before becoming a demon.) Also, pillars still may think it is not safe.
I don't know why Nezuko didn't train herself to use breathing techniques or because of her bamboo piece in her mouth(Just kidding!). But Yes, demons can learn breathing techniques.
Just like Upper Moon One "Kokushibo". He can use Breath of the Moon. Link: https://kimetsu-no-yaiba.fandom.com/wiki/Kokushibo
In my opinion, demons can't train and get stronger the way humans do. Tanjiro needs to train to get stronger; she doesn't. Also when she sleeps it's not a normal "let's sleep at night" kind of sleep, but more of a "I'm going into a long coma" kind of sleep. She can't train at day and sleep at night; that's not how her recovery process works.
It's not known yet in anime that whether demons can develop breathing techniques. But it seems like that cannot develop breathing techniques because demons don't feel the way like humans do. Tanjiro as a protagonist in the series keeps on training and improving his breathing technique to save and bring Nezuko back to her old form. On the other side, Nezuko, being a hybrid type (not completely human nor a demon) recovers from injuries by sleeping and the same technique goes when she is asleep. In order to protect her sister he keeps getting stronger to bring nezuko back to her old self.