Really cool, man
You were still arm-pumping the jab at times, but considering that monster right hand it set up, who the fuck am I to complain?
The reason you're leaning back is that you're smothering your range a bit. If you're at a proper punching distance, you will feel very comfortable throwing the weight forward and keeping that hunch-shouldered posture, ready to react and defend. If you're too far forward, you can't get your proper posture while still leaving enough room to swing, so you're leaning back to compensate. It's just a matter of getting comfortable with your ideal range, which will come when you get that jab fully extended and using the hip and shoulder shuck to add some range and pop. If you're arm-pumping it, though, you're going to cut your range and start smothering yourself.
Happy to see some body work but don't lead with it unless you're fighting in a phone booth or clinch situation! That guy was shitting his pants from the opening bell, but a competent counter puncher will make you pay for that. A little trick: a stutter-step jab sets up a 3b nicely; rather than just leaping with the 3b, drive hard off the back foot, pop buttcheek forward, shuck shoulder as you normally would for jab, but angle yourself off with the drive slightly diagonally to the left, in a position you'd normally be after slipping a 2 to the outside. It's actually throw in "mid-air" for the nanosecond that you're "in flight". The weight will end hard on your front leg, and the exact moment that it does you throw it violently back onto your rear leg and rip that 3b. From here, you can follow up with the right hand and thread together a combination, or angle off, as you're already off-center, you'll make him whiff his counter. It's really frustrating when your opponent can blind you with a jab, pound some vigor out of you with a 3b, then make you whiff a counterpucnh. You feel totally incapable of anything
Might want to change up your head movement a bit. It's good that it's there, but you're a little teeny-bop bouncy and back-and-forth. Try and mix in patterns and variations, and play with your rhythm or you may be timed. Also, don't be afraid to back off and use a little more subtle motion sometimes else you'll burn unnecessary energy... though it doens't seem to be a problem when you're a conditioning beast who KOs their opponent in the first round. The idea is to have your head a moving target sometimes, and other times jsut to have it MOVING at all because it's easier to roll into a more dramatic slip once you've seen the punch coming if you're already moving rather than going from a dead stop.
Great job overall I'm just being a dick over little things. Enjoy your time off and message me for any boxing-related stuff you need, schools been a bitch but I'm always lurking around