I know this is kind of already in the mod but I feel like it can be way better, something to look at for inspiration is the original Improved First Person mod from back in 2013
What they did was force the bow to be in the left hand and be pulled back with the right hand (like how most people use the bow)

as for how you could fix the crossbow, maybe remove the breathing arm animation and force the left hand to stay under the crossbow.