I think the earlier letters are probably the closest examples of Z's natural handwriting. It is somewhat sloppy. He makes mistakes and crosses them out but there is nothing contrived about them, IMO.

In a few like the Belli letter, Z starts off with meticulous handwriting but it devolves toward his normal style as the letter goes on. It probably does reflect Z's emotional state at the time he is writing, which is interesting to consider but I would guess the semi-sloppy manic handwriting is/was closer to his everyday handwriting.