I've seen this done before, and one of my suggestions for it would be to split this into at least 2 classes, one for Kido focused Shinigami, and the other for combat focused ones.

Also, their weapons should be considerably better then they are as written. The weapons (at least) should become +5 by 20th. On top of any special abilities granted.

Maybe instead of a pre-set list of abilities, you should have 'talent trees' similar to that of D20 modern and similar games. Or at least optional starting builds, eg. one with little to no spellcasting, but full BAB; and one with half BAB, but lots more casting ability. Also, I don't think they should get healing normally. probably as a PrC, as only one of the squads has shown any healing ability at all.