Lengthening and shortening the stride really works their back end
Lunging , hillwork , and lots of Circles and surpintines !
If you have access to a hill walk up and down the hill to build up chest muscles or add trot poles in to your lunging
Agree with Hannah totally I always use my Pessoa system! If you don't want to spend that much money they have off brand systems that work just as well
I have a 4 yr old and I'm constantly lunging her in a pessoa system which that really helps bring them underneath themselves and build up that hind end as well and neck. Obviously those are a little more on the pricey side so you can also use side reins which will do the job just as well. Just make sure you are using them correctly because they are easily misused. Good luck! 😊