I'm going to mostly comment on your working your way towards an automatic release, which I think is an appropriate goal for your level of riding.
I feel from looking at these photos that you're skipping an important step in progressing to an auto release. Normal progression is from a short crest release (hands 1/3 to 1/2 up the neck, slack in the rein) to a longer crest release *while maintaining some contact* then dropping your hand and arm down from the exaggerated broken line of a crest release towards the straight line between bit and elbow, with contact of an auto release. A true auto release has the contact being exactly the same throughout the take off flight and landing. There are also degrees of auto release.
In Allison Finch's wonderful avatar photo, she's using a modified auto release, taking some support from the horse's neck, but pretty close to a straight line and an appropriate soft contact.
The most difficult auto release to execute successfully is one with NO support form the neck, and hand and arm following the head and neck in flight with the no change in the contact.
In these photos, it looks like you've tried to go directly from short crest release w/o contact to auto release, and you've missed the whole feel of maintaining the contact and having the horse pull your hand and arm forward as they take off.
So I'd recommend backing up one step, and work on a long crest release and *maintaining contact* while taking some support from the neck, and then gradually dropping your hand and arm down towards an auto release.
Now, some photo by photo critique -
1.) Slight pivot on knee, good mechanics of the release, but no contact.
2.) Arm hanging straight from shoulder, no bend in elbow, therefore, no contact. Same lower leg.
3.) Better lower leg, this is the kind of long release I'm talking about, that may be more appropriate right now.
4.) Like your body position in this one. If your reins were shorter, and you were maintaining a little contact, this would be ideal.
5.) Good body mechanics here as he overjumps dramatically, hand and arm too low, no contact, reins to long to effectively steer.
6.) Lovely. Take the slack out of the reins and maintain contact here, and you'd have a very polished, effective picture.
9. and 10. are more where I think you should be. This is halfway between the long crest release and the true auto release. I think your horse actually prefers some contact in the air - he seems a little better put together in these photos.
In general, if you want to give him total freedom of his head and neck in the air, meaning, loose reins, use an elementary release - short crest release. If you want more control and influence over him, and a little support for you, use a long crest release and maintain contact in the air. If you want maximum influence and control, use a full auto release, but that means don't throw the contact away at the same time.
Finally, your lower leg is inconsistent and needs a little strengthening. But when it's good, it's quite good, and good gosh, are you ever leggy! Terrific riders conformation, but learn to use that lovely long leg to your advantage.
Excel looks like he's just loafing over this little fences, barely paying attention, overjumping without a care in the world. I'd like to see him do more grids, gymanastics and technical distances to sharpen him up. While I'm glad you're not jacking up the fences because he's talented, I also wouldn't want him to become bored.
Good luck, and keep posting photos! I love to see your progress!