I agree with your comments and the degree of erotic tension is well produced. It was an unusual punishment position having the girl straddle the horse, and I like to think the sensations of the wood between her legs as she is punished is something she may look back on later and enjoy in her private time, if not at the time of the punishment as the paddle, with its interesting cross hole feature stings snd marks her buttocks. I assume the circling of the punishment horse by the Mistress after each stroke was to allow the full effects of the stroke to register and throb, before applying the next. As enjoyable as the face shots were, I would have liked to have seen more bottom shots, so the balance could have been improved there, and the pixilisation of the private parts is an unfortunate, superfluous feature of many Japanese films, which does little to hide what's happening and what the consenting viewer has come to see; merely interfering with the full enjoyment. I would also like to see more from this producer, who certainly displays considerable potential for capturing the mood and power of ritualisation.
I think this is extremely erotic. The strong institutional ritualised procedure and intense humiliation are vital. The expressions on both girls' faces are excellent. Of course it could be improved on. The process of positioning her over the tressle and then taking down her panties was missing. And of course it would be much better with her anus and slit fully displayed during discipline. But I would like to see more from this director, or film series.