Lighter texture for chocolate pie
This is a basic chocolate custard filling.

The one thing I would suggest is changing out the whole eggs for egg yolks only, at a ratio of about 2-3 yolks per cup of dairy, so 3-4 egg yolks in this recipe.  This should yield a more tender and silky custard, as the whites tend to contribute a more rubbery or resilient texture.

Even so, the original proportions do not look that far off.   A custard pie like this one should be able to be sliced when cool, and the slices should hold their shape without slumping.  That does require a certain amount of strength in the custard.

You might wish to look for recipes based on a mousse filling, which is not baked in the shell, such as this one from Epicurious.