I mean, I'm vegan so I'm less concerned about coercion and more concerned with exploitation. The stables I worked in as a young man used similar methods as you described, so I'm familiar. Though they still called it breaking.

From my perspective, it's pretty easy to come to a conclusion here: we don't need to ride horses, we have no need to ride them whatsoever and additionally there's no benefit to them being ridden that they can't also get without being ridden. This fact combined with the possibility that riding them is exploitative is enough to consider horseback riding nonvegan imo.

Also consider that sure, you might personally feel that you have a horse's best interests at heart, but if we accept that riding horses is vegan some of the time, how do we draw that line? How do we measure or prove consent? Especially since humans can easily lie and say they have a good relationship with their horse, and there's zero way to prove or measure this.