Psychological depth in Tolkien's characters

[Eurytus]

1- JRRT wanted, and rather intensely at that, to publish the Silm in conjunction with the LotR. This would have given, for instance, literally thousands of years of back history to a character like Aragorn, we would already understand what 'the Heir of Isildur' means in a far greater sense had the above publishing happened as JRRT wished. This would in turn influence our [or at least it would my] reading of his actions in FotR.

However this did not happen. So can one fault JRRT for not putting all of Elrond's or Galadriel's or even Sauron's backstory
into the LotR ?

Sorry but I can blame him. A review of his correspondance shows that he knew well in advance that the Silmarillion was extremely unlikely to be published and so it remained up until his death.
He tried numerous times to palm various works off as a possible sequel to the Hobbit and was often rebuffed.

So yes, a work should be able to stand alone. If there is shallowness to a character in LOTR it is absolutely no defence to say, "but I've got tons of backstory for him in my unpublished scribblings!"

And further, backstory and history does not necessary give psychological depth. Take the interaction between the three leads in Jaws. Little to no backstory is needed. Little to no history. Interaction leads to depth, which is as it should be.

Hence the laughable shallowness of Legolas's (aka the shop mannikin) character.