Currently it is held like if it were a bow or a tool (at least that's what I noticed in the 1.12.2 version), I think it would be better to change this to look more like you're holding an actual crossbow like in vanilla and Tinker's Construct.

Good suggestion and also already on my offline TODO list - I just didn't copy everything that is there to GitHub issues :)
However, this will definetly be done in the next update!

Tried a few quick and dirty things until I noticed that most of the cosmetic stuff for the Vanilla crossbow is pretty badly hardcoded within various classes throughout the whole codebase...
Will postpone doing this properly for now. However, I will make holding it at least a bit better :)

Okay, after trying some stuff I need to say that this is way harder than I thought it would be. I will postponse this entire issue even further, probably after the next release