I really want to get rid of Seelah - What's a good tanky replacement class, merc-wise? I guess with the power levels being what they are the classic heavy armor/shield approach won't work.
Almost any class can become tanky, if you are willing to use buffs. Since you're playing a Sohei, you can boost your horse's AC to make him unhittable without having to waste a character as a "tank" (your characters should really do more impactful things than soaking up hits). Make him Bulky (or whatever the archetype that gives armor proficiency is called) and cast Shield of Faith, Barkskin, Cat's Grace, and Shield on him. Take Impossible Domain: Nobility on your Cleric for the Insight bonus, and you should be set. So I guess my answer is an Alchemist, since it allows you to cast both Shield and Barkskin on your mount. Make him a Vivisectionist if you also need a melee fighter or an Incense Synthesizer for the alchemical bonus to hit.
Personally, right now I'm using a Bard Seelah as my "tanky" character. With a light armor, a shield, the Dodge feats, and Crane Style, she is sitting on AC 40 since level 6.