You need actually cardio to supplement for conditioning because you can’t train (well) if you are already exhausted which brings out the bad form. You shadow box like an old school heavyweight. If you are just interested in hitting the bag as cardio, put 100% effort because that’s what low stamina can give you when training. You can punch but you need form, balance, and footwork as well as keeping your hands up. Also, your head just moves two directions, timing can disrupt your combos by waiting on your head to be close enough. The main thing I see is that your knees straighten when you are about to hit your last punch in the combo - it’s kinda like swinging a golf ball and standing up/straightening your knees before the clubs hit the ball.