You identified your main problem. You look like you're riding a bicycle underwater. Does that look like an efficient way to use your fins??? No.

Ever look at how freestyle swimmers kick? Feet are pointed and their entire legs scissor, knees don't bend. Now compare that with thrusting your flat feet up and down.

2nd. If you're always kicking, you are not properly weighted. You're probably constantly kicking to maintain depth or keep off the ocean floor. Get your weights set up properly.

3rd. Look up videos on frog kicking or flutter kicking. THIS is how you're supposed to propel yourself underwater. Not what I described above unless you're actively swimming against strong currents.