Anyway, I just tried flattened and it accepts documents, but doesn't alloww querying them the way we want, so I'm working on a different solution.

We tried to change the mapping type of the field intrinsic_metadata from nested to flattened as you have suggested, we have now a new error related to the huge size of a description.
ES can be configured to accept bigger fields but I'm not sure it's relevant regarding the description field content.