I lived on the back of a horse for five years as a teen. As a result, I didn't kiss a boy until I was 18. lol. If my mother had been more supportive of a horse career, I likely would have gone that direction. Instead, well, let's just say that if she had to do over again, she says, she'd have bought that horse trailer and made sure I was at every competition and had all the support I could want. lol
I turned out fine, really, in the end, but I agree you are an incredible dad to be so supportive.
The list of things you're already paying for are basically it, excepting show fees, the extras that do pile up: medical supplies, shampoos, fly sprays, new blankets when the old ones get trashed, and a medical fund. Horses LOVE to hurt themselves. It's a hobby. Set aside an extra $100 a month (up to say, $500) then an extra $50 a month. Just in case. It'll also help cover the infrequent things, like teeth floating. Oh... and worming. It's not that expensive, but it is a constant.