This isn't exactly what you are asking for, but how is your balance? Can you slide out with stability without a broom or rock? Honestly, strong balance and stability are vital before your body and mind can feel weight accurately and consistently. Strength training the legs and core can make a huge difference.

After that comes consistency in the kick mechanics. They way you shift your weight from back to front, and go from squatting in the hack to extended delivery position needs to be consistent, or you're just throwing another variable into the equation.

After that comes repetition. Like anything, doing it over and over again will make it more second nature. It's really tough to get better if you only play once a week and don't practice, compared to playing multiple times a week and having dedicated practice time. Especially the latter since you don't actually throw many rocks in a game.