PS: about this part:
> Once per round, the killed PC’s player adds their CHA bonus (or a d6, their call) to any roll made by another player, before or after the roll.
I like the "spirit" boon bonus to the still-alive allies, but I think that the d6 part doesn't to justice to the CHA ability. You already spelled out that "Wisdom and Charisma are often ignored as “dump stats” in favor of abilities with more combat utility, but now feature uses that should help keep PCs alive." Removing the d6 part will make CHA shining a little bit more.
PS: did you tested the game with at least 3-4 characters on the field? If two or three of them dies during a big showdown, that means that the last one actually can add 2 or 3d6 to one roll, every round, hypothetically boosting it a lot. Can he sum all those spirit bonuses to the same roll?