First of all, you have a lovely seat and legs. I really commend you on your position, it is wonderful!
Just a few nit-picky things I noticed:
Your hands sometimes have a tendency to work backwards and too wide. In the halts and on the one extended trot picture you are working too much backwards, remember that the half halt is a combination of aids, with the hand being least important. Try to ride the downwards transitions more with your seat and back and bring the forehand up in the half halt, not back. Also in the bending movements and some of your corners I noticed that you raised and indirectly used your outside rein. This is not correct, instead the outside hand should be down and the outside leg should push the horse around the inside leg, with the inside hand coming slightly up to flex and bring the forehand upwards.
In the walk, you do not so much slouch as crouch in your seat, and aren't sitting up properly and riding with the walk motion. You also don't ideally ride your walk half pirouettes. You kind of just shift your hands over and twirl your horse around without riding the steps individually.
Overall, your horse is not as collected as he could be, and I think this is because you aren't riding the collection, and instead you are just always pushing forward. He is definitely active enough that you can start closing him up more with your seat, legs and "up" thinking half halts. I noticed this especially in the trot, he gets long and his poll gets too low, especially in the half pass. The canter is much better and the walk really needs work. He almost runs away in the walk, you need to get it so that you are riding every step. A good exercise is to halt and take two steps forward and halt and take two steps forward, until he is on your aids and really thinking about his walk. Then in the pirouettes you can really start riding him more.
Just one other thing is in the pictures where others have said you are "slouching" and your horse is behind the vertical, I'm assuming these are warm up pictures. As a side note to others, this is not slouching. This is compressing your body so that you can reach your hand far enough down to stretch the horse. I have the same problem. Outside of the warmup this would be incorrect, but in this situation it is perfectly alright because there is not slouch, it is more of a reach. Her abs are still toned and she is still sitting in the saddle.
Also, yes the horse is behind the vertical. To fix this, don't put your hands so wide! Just rest them on the withers and ride the horse to the contact instead of pulling him into it. He will do the correct stretching if you ride him correctly to the contact.
I also really like your horse, my horse's grandsire is Comeback II and I can see a really big resemblance between mine and yours! Kepp up the good riding!
PS. How old is your horse?