For us, showing is a bit of fun, and something different from the eventing that is not only a good day out, but is not stressful or taxing in the same way as eventing is. We'll never win championships, but we enjoy ourselves, and we aim to get better every time.
I see what you mean about Sandie's movement - I'm no American Hunter rider, but I know a bit about judging horses' movements from our showing here, and you can most definitely train to improve her movement if you want to. You say she likes a long, low outline - use that to lengthen her back and encourage her to round and engage, and get her hindquarters powering her more. Once she's more engaged, the collection into the smaller paces the APHA seems to want will be much easier. Long & low is a great way to build up self carriage too.
Best of luck, and don't let the craziness of the showing world get to you. I've seen not only completely lame horses get pulled in first, but perfectly sound only on the forehand horses dismounted as lame - the judge calls it, and unfortunately not every judge is the same (or even sane).