You can do a 18/18 Yumi build and grab some PS, or 21/18. 15/24 with a horn/tatar/nomad is always solid. Horses are totally up to you, I personally hate desert and arabian because they're so damn weak. Rouncey actually works just fine, it's more about being smart with how you ride rather than the raw stats on the horse. Keep in mind most HAs can't really do much so they just aim for your horse, Mokuchin being a prime example of that. With 15/24 you'll really be wanting to aim for the head to make it work, getting that build to 31+ affords you some more riding.
I rufuse to use the yumi based on it looking like total shit and having a slow ass missle speed. Granted it works well I can't do it. I'd suggest making a STF and trying out 15/24 and then a higher PD build and seeing what you like more. The difference between 3 and 4 HA is pretty noticeable in terms of accuracy, and while 4 reduces the penalty, 6PD still gives you a damage edge (plus the yumi being 2 more dmg than horn).
