For strength use weights as you are doing.
Big weight and low reps build strength and power.
For cardio you can't beat running, it is still the "King of exercises", and best done on the grass over the Downs or somewhere in nature where the ground is soft and the air is clean, and the view is more inspiring.
However, a bag workout, a good shadow session, especially kicking, and sparring, will all get the metabolism up and work cardio.
For heavy bag, it's good to do that for power in technique, but too much retards speed. Balance with shadow boxing and practice your techniques on the air, against imaginary opponents, or lined up to target. Line up on the bag without making contact.
The first 3 attributes to a technique are form, speed and power. Form is the most important. Getting this right makes it easier to add speed and power as you progress.
Distance and timing are the most important attributes, developed through drilling.
Co-ordination and balance go hand in hand, and the drills develop these.
The best thing to do is keep as close to the partner drills and sparring that we do in class as you can while working solo, in shadow or on impact kit like the bag. It's the reason why kata were invented.
Keep your outside class practice as close to your in class practice.
