correojon look at how existing games do it.
look at god of war, the axe hangs in the back. other games just have the weapon float over the character.
for sheathing into a scabbard you need two models, one of the sword, and one of only the hilt.
parent the sword to a boneattachment3D for the hand, parent the hilt and scabbard to a boneattachment3D for the pelvis or spine.
at the point where the sword enters the scabbard in the animation, hide the sword, then show the hilt. when the hand reaches towards the hilt in the animation, hide the hilt and show the sword.
it's that simple.
the other way is to animate the weapon WITH the character, this is how GOW does it, it is not scalable and you can't use retargetting because the animation has to be tailored to the character.