Well, I can see what you mean about her conformation not being the most conducive to her reaching well under herself and arching and reaching forward with her neck (kinda short).
But, she moves very rythmically, and though she doesn'thave much swing in her back, she pushes decently off the hind end and is balanced enough that when you asked for the halt, she was able to step into it promptly without the slightest loss of balance.
What you can do is do LOTS of transistions. Working the hind end means acceleration and deceleration. So, go up to trot, trot , up to canter, back to trot, to walk, and so on. Do some quick starts, from time to time, too. With deceleartion make sure they are good ones, and that she is promptly but not startled. So, prepare her and keep a bit of leg on even as you firm up your core to signal a stop.
Do more turns and smaller circles , and then work into doing up and down transitions ON that circle. This is where she will have to engage her stomach muscles, and doing this will build topline and help her lift the base of her neck.
She looks like a really sturdy mare and I bet you have some really nice rides together. It's nice that you are thinking of helping her move in a way that will keep her in the best balance and fitness.