Will the meaning change if it was "must" instead of "should" here?
Answer from a Native speaker
It would change ever so slightly! "Must" would work really well here, actually. But there is a slight difference in nuance in this context! In this sentence, "must" would mean that you're certain or expecting the other person to be happy. "You <should> be happy with this" is, like, you've done enough work to be satisfied, so be pleased about it, and you may not think the other person is satisfied yet. Ex: You should be happy with the work you've done. You've worked hard. = You must be happy with the work you've done. You've worked hard.