When working with a younger horse (even if you have a few years of dressage experience under your belt) it is absolutely essential to have a dressage coach. It is very easy to teach them bad habits, and is going to be extremely difficult to move up the levels without having a knowledgeable coach on the ground and that can get up in the saddle to teach the horse.
As a rider, it is very difficult to teach the horse things that we ourselves do not know, which is where the coach comes in.
Tell it to your parents like they've spent all the money on a horse, now in order to keep its value up they need to invest in training it properly. Although there are no coaches in your area, I am sure there are regular dressage clinicians that come to your area every 4-8weeks. Try to get into these regular clinics and just use your eventing coach for lessons "in between". Also try to have your eventing coach attend and watch your dressage lessons to keep the teaching consistent.