Not that you're wrong, but here is my personal opinion:
-Poseidon is good, really. Atk (and def) Debuff is the most valuable thing a kamihime can bring. Her dot is really good early game, and will deal good amount of damage overall. Ofc its less impressive than Shiva's spike damage
-Forget Rusalka. Gabriel incoming in few days and is pretty much a guaranteed kamihime.
-The worst thing to do is to choose a soul depending on weapons. The only thing affected by weapon's rarity is your soul's burst damage, which is only a small part of your damage. The only soul based on burst damage is Siegfried, and as you said, glaive drop from raid boss.
Better than that,
next regular event is water, and will bring
SSR glaive; It will also bring SR sword and gun.
It means that with this event and raid boss, you will have at least SR weapon for every single soul in the game ^-^