I agree that you have a lot of good things going for you. But since you asked for input, here's what I see:
In all three videos the horse is lacking in impulsion, and this is making a bit hard for you to post and to connect him to the bit.
In video one, it looks as if your stirrups are maybe one hole too long. Also, I am wondering if the saddle isn't a bit high in front. I mean, there's a hint of you trying to deal with a saddle that is oriented uphill; your legs go forward, and consequently your upper body must go forward, too, in order to maintain balance. What kind of saddle is this?
The horse, in video one, looks more relaxed and balanced and seems really happy to be with you. (I know that may sound woo-woo, but it means a lot)
Video two you seem to have a bit better leg position, but mostly it is like video one.
Video three, Dexter seems irritated, hollowed out and often sucking back. I'm wondering what could have caused that.
As for you, in all videos your elbows are out too far from the body. I noticed that you tried to give an opening rein cue at one time (can't remember which video) by pulling your whole arm off your body. You can do the same cue by simply swinging your lower arm awary from your body, but keeping the upper arm attached to your rib cage.
I think a lot can be improved by keeping your arms in, dropping your shoulders (think 'elbows connected to my hips'), keeping thumbs on top and hands closer together. AND, getting Dexter to wake up and put some oomph in his trot!
Your canter seat was really quite nice. I wish I could be that quiet.