Psychological depth in Tolkien's characters

[The Saucepan Man]

On one side, there really isn't that much to Legolas. On the other side, we have the interesting case of someone that occupies a crucial niche in the narrative

Precisely. The fact that Legolas as a character lacks psychological depth doesn't make him any less of an integral character in the context of the story that Tolkien was telling.

As I have said many times on this thread, the fact that many of Tolkien's characters lack psychological depth is not a criticism. It is simply a comment.