I agree that it would be nice to see something of Brandon in cheerful spirits. I would go further and say that it would be good to see something more substantial of his developing relationship with Marianne.

Over the course of the novel we get to know both characters pretty well individually, so that their eventual happiness is at least credible when you think it over. However, we see next to nothing of any relationship between them. I trawled through the novel and here's what I found:

Brandon smitten from the first, doing a lot of yearning and worrying and being unobtrusively helpful. Meanwhile, Marianne's attention is somewhere else entirely. I don't think we see any actual conversation between them at all - if there is, it completely escaped my notice.

After learning of Willoughby's infamy Marianne stops avoiding the Colonel when he calls, and voluntarily speaks to him with compassionate respect. (Ch.32)

In Ch.46 following her illness, Marianne invites him to visit her; she is impatient to pour forth her thanks for bringing her mother to Clevedon, and we see her holding out her pale hand. On leaving Clevedon she bids him farewell with the cordiality of a friend. We learn that she knows she can borrow books from him.

When he comes to Barton (Ch.49) we are told of the kindness of her welcome.

The whole affair is wrapped up in precisely twelve irony-laden sentences towards the end of the final chapter, (which is less than the account of Lucy and Robert's 'courtship' and marriage!).

And that's it! As an account of a relationship it just isn't enough to engage my interest as I read, no matter how fine a fellow Col. Brandon may be, because we don't see enough interaction between them.