It’s like the D850 and D500 inheriting the D5’s AF system yet they’re not quite as good. There’s other things that contribute to AF speed and accuracy, the pro bodies will likely have better components.
I’d be surprised if the A9 is “much” better, but it will be better. The A9’s system is probably overkill for equestrian, in fact it’s probably overkill for most things. I wouldn’t get too hung up on it tbh, with Nikon the D750 and D500 they have been more than adequate for you and at no time do I recall you thinking you need the D5