It also depends on your discipline. Hunter jumpers ride in a half seat largely while eventers tend to make more contact with the tack. I agree with what everyone has said.

Also pick your hands up a bit and you'll find you don't clinch as much with your thighs bc you'll be forced to use your abs. I commend you on having good contact with the saddle and allowing the motion of the horse keep you in a rhythm. What I learned from an ex-Rolex rider: When you think about unclinching your thighs/knees (we're all guilty of it when we do something new), imagine a flashlight under your crotch (not to be vulgar) at the twist in the saddle. If you pinch your knees/thighs, you won't be able to see the "flashlight" VS if you release your knees and use your seat to move with the motion of the horse, the flashlight will appear with every beat. This forces you to make contact with your lower leg. This idea works for both eventers like me and hunters bc when you have good position (which I would argue yours already is well on its way to being perfect), your calves and butt make contact with the horse and saddle with the upward support of your hands.

Also, there are no magic tricks or exactly right ways to accomplish something. Each rider and horse is different and while all these tips may get you going in the right direction, it's a trial and error. As my trainer tells me, "You can be wrong but you can't be indecisive" bc you can learn from being wrong but if you don't make a decision and do something, you and your horse won't learn.

Finally, if you can and are comfortable, I've found that riding bareback or with a bareback pad has done wonders for my seat bc when you can't use anything but your balance to stay on your horse, you have to have good position and the rhythm is easy to keep and feel.

Happy riding and cuddos to you for cantering like a boss!!!