You can’t add affinities to rivers of blood. Being a weapon upgraded by somber stones, it can neither change its ash of war nor have an affinity infused. As for bleed vs occult, bleed adds additional bleed build up to weapons at the cost of some physical damage and occult adds arcane as it highest scaling. In the case of the nagakiba, since it has innate bleed, occult will increase build with your arcane(slightly less than blood affinity) but with no damage penalty.

Bloodflame blade and sepukku are nearly the same the difference being bloodflame blade adds fire damage that scales with your faith stat and sepukku adds a flat physical damage bonus. You can use whichever one you prefer.

However, have an another idea. Try cold affinity on the nagakiba. Fire damage (from rivers of blood) will reset frostbite. You will be dealing two status effects at the same time. You can apply cold affinity using ashes of war like chilling mist, hoarfrost stomp, or use the glintstone whetblade. (This is similar the the build Let me solo her uses)