I'm a beginner dressage rider! It's seriously amazing and it is truly rewarding. It feels so wonderful to achieve the results for which you are working. You're absolutely right, it will help with both of your issues. From my understanding, hunter riders ride with a more forward seat. As a dressage rider, you will learn to sit deeper into the seat with a more upright posture. Your canter, instead of in two-point, will be deeper, you'll be moving with the horse using your hips. I think it'll be great for you to learn dressage because you'll know two kinda different disciplines and use universal skills in both. God bless!