I am not working on an avatar. I am working on highly animated fictitious humans, including synthetic voice. I go through Blender, and get the Kinect V2 skeleton using the MakeHuman Blender add-on. I wrote the part of the add-on which does this.
I have since modified the skeleton conversion code to not swap out the hands, but keep the original, high detailed finger bones, but never pushed this into the add-on. There is a 2nd version of the add-on, but I have not gone to it.
I think you are going about this from the wrong direction. You should look at https://forum.babylonjs.com/t/vr-avatar-support and find out what the requirements are like bone needed & names required, morph targets needed, etc.
From there decide where you are going to source things.