Some various changes, originally I was going to keep it to frame timing changes (for example, I sped up the windup at parts and removed the pause at the ending) but I then went a bit further.
First, I added animation smearing to the recoil to simulate motion blur, which you can see pretty easily. The motion you had for the arm was pretty much on point, so I didn't faff about much further with it beyond adding a bit where the arm recoils backwards a bit, for more of a hefty effect to it. It's hard to see, but there's a frame where I added some of the arm color over the backpack.
Finally, I shifted his weight backwards onto one leg, then forwards again, before he finally catches his footing again from the punch!