I'm having trouble developing balance/power with any kicks waist high or above. I suspect this is due to poor flexibility. What are the best stretches for developing high kicks? Thanks

Add Resistance. Raise the bag. Triple the reps.

You don't really need flexibility to be able to kick waist high. It's a matter of lower and mid back strength, position and hip flexor strength.

Get a stable chair and hold onto the top of it, seat facing away from you, as you kick. This keeps your body upright.

Put a piece of tape on the bag where you want to connect with your lower shin and try to hit it. If it's too high, put another piece below, but focus on accuracy and upright body position first.

Only stretch, if you must, after you've raised a good sweat and body is heated up.

Do the splits as far as you can and then get somebody to push down on your shoulders until your balls get squished. If you hear your groin tear don't panic, that is what you want to happen.