I think the biggest thing will be to get more impulsion out of the horse. You have to work too hard to post because the horse isn't giving you enough to post off of. I think your position is actually not so bad. You look reasonabley centered, have a decent elbow to bit line for much of the time, and I can see that you are working to keep you lower leg under you . I do see it "pulse" outward with each time you push up for the post. Maybe if you were able to put a bit more of the inside of you calf against the hrose, which means turning your toes to be more parallel with the body of the horse, instead of pointing outward, as one tends to do when jumping. Think of your weight going down the outside of your thigh, calf and past the outside ankle bone , through your heel and PAST your stirrup, so that your heels are pointing toward your horse's rear hooves.
I thought your arms were back just fine, except a bit more anchoring of the outside elbow will help to steady the outside rein.
Carry a crop a few times and ask him to move out. Whnen he does, take up some contact and "string" that energy between your leg and your hand so you have something to post off of.