I myself learned modding a couple weeks ago and did not have any trouble following the suggestion from dieseben07 which was: "cancel the event and do the rendering yourself". If you want to get anywhere with modding, you need to learn it properly.
On PlayerRenderEvent.pre, take the arm model, and set the visible field to false. In RenderPlayerEvent.post, set the field to true, then adjust the x, y, z fields of the arm model (the rotation points). Y is either 20 or 17.5 if crouching (i believe). Use some trigonometry to calculate the rest from the player's rotations. Then you have the xRot, ... fields which you can modify. You might need some math for that too.
After you're done modifying the pose, call render for the arm model. It takes the matrix stack from the event, a vertex builder, ... you can see an example of using this from the vanilla rendering code. I actually learned how to do this by studying vanilla code. You should too! All the code you need is there.